Chapter 5. MySQL Server Administration

Chapter 5. MySQL Server Administration
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Table of Contents

5.1. The MySQL Server

5.1.1. Option and Variable Reference
5.1.2. Command Options
5.1.3. System Variables
5.1.4. Session System Variables
5.1.5. Using System Variables
5.1.6. Status Variables
5.1.7. SQL Modes
5.1.8. Server-Side Help
5.1.9. Server Response to Signals
5.1.10. The Shutdown Process

5.2. The mysqld-max Extended MySQL Server

5.3. MySQL Server Logs

5.3.1. The Error Log
5.3.2. The General Query Log
5.3.3. The Update Log
5.3.4. The Binary Log
5.3.5. The Slow Query Log
5.3.6. Server Log Maintenance

5.4. General Security Issues

5.4.1. General Security Guidelines
5.4.2. Making MySQL Secure Against Attackers
5.4.3. Security-Related mysqld Options
5.4.4. Security Issues with LOAD DATA LOCAL
5.4.5. How to Run MySQL as a Normal User

5.5. The MySQL Access Privilege System

5.5.1. What the Privilege System Does
5.5.2. How the Privilege System Works
5.5.3. Privileges Provided by MySQL
5.5.4. Access Control, Stage 1: Connection Verification
5.5.5. Access Control, Stage 2: Request Verification
5.5.6. When Privilege Changes Take Effect
5.5.7. Causes of Access denied Errors
5.5.8. Password Hashing as of MySQL 4.1

5.6. MySQL User Account Management

5.6.1. MySQL Usernames and Passwords
5.6.2. Adding New User Accounts to MySQL
5.6.3. Removing User Accounts from MySQL
5.6.4. Limiting Account Resources
5.6.5. Assigning Account Passwords
5.6.6. Keeping Your Password Secure
5.6.7. Using SSL for Secure Connections

5.7. Running Multiple MySQL Servers on the Same Machine

5.7.1. Running Multiple Servers on Windows
5.7.2. Running Multiple Servers on Unix
5.7.3. Using Client Programs in a Multiple-Server Environment

End of Product LifecycleActive development and support for MySQL database server versions 3.23, 4.0, and 4.1 has ended. However, for MySQL 4.0 and 4.1, there is still extended support available. For details, see http://www.mysql.com/company/legal/lifecycle/#calendar. According to the MySQL Lifecycle Policy (see http://www.mysql.com/company/legal/lifecycle/#policy), only Security and Severity Level 1 issues will still be fixed for MySQL 4.0 and 4.1. Please consider upgrading to a recent version (MySQL 5.0 or 5.1).

MySQL Server (mysqld) is the main program that does most of the work in a MySQL installation. This section provides an overview of MySQL Server and covers topics that deal with administering a MySQL installation:

Configuring the server
The server log files
Managing user accounts

5.1. The MySQL Server

5.1.1. Option and Variable Reference
5.1.2. Command Options
5.1.3. System Variables
5.1.4. Session System Variables
5.1.5. Using System Variables
5.1.6. Status Variables
5.1.7. SQL Modes
5.1.8. Server-Side Help
5.1.9. Server Response to Signals
5.1.10. The Shutdown Process

mysqld is the MySQL server. The following discussion covers these MySQL server configuration topics:

Startup options that the server supports
Server system variables
Server status variables
How to set the server SQL mode
The server shutdown process

Note

Not all storage engines (also known in older versions of MySQL as “table types”) are supported by all MySQL server binaries and configurations. To find out how to determine which storage engines are supported by your MySQL server installation, see Section 12.5.4.8, “SHOW ENGINES Syntax”.

5.1.1. Option and Variable Reference

The following table provides a list of all the command line options, server and status variables applicable within mysqld.

The table lists command-line options (Cmd-line), options valid in configuration files (Option file), server system variables (System Var), and status variables (Status var) in one unified list, with notification of where each option/variable is valid. If a server option set on the command line or in an option file differs from the name of the corresponding server system or status variable, the variable name is noted immediately below the corresponding option. For status variables, the scope of the variable is shown (Scope) as either global, session, or both. Please see the corresponding sections for details on setting and using the options and variables. Where appropriate, a direct link to further information on the item as available.

Note

This table is part of an ongoing process to expand and simplify the information provided on these elements. Further improvements to the table, and corresponding descriptions will be applied over the coming months.

Name	Cmd-Line	Option file	System Var	Dynamic
Aborted_clients				Yes	No
Aborted_connects				Yes	No
abort-slave-event-count	Yes	Yes
allow-suspicious-udfs	Yes	Yes
ansi	Yes	Yes
autocommit			Yes		Yes
back_log	Yes	Yes	Yes		No
basedir	Yes	Yes	Yes		No
bdb_cache_size	Yes	Yes	Yes		No
bdb-home	Yes	Yes	Yes		No
bdb-lock-detect	Yes	Yes			No
- Variable: bdb_lock_detect			Yes		No
bdb_log_buffer_size	Yes	Yes	Yes		No
bdb-logdir	Yes	Yes	Yes		No
bdb_max_lock	Yes	Yes	Yes		No
bdb-no-recover	Yes	Yes
bdb-no-sync	Yes	Yes
bdb-shared-data	Yes	Yes			No
- Variable: bdb_shared_data			Yes		No
bdb-tmpdir	Yes	Yes	Yes		No
big-tables	Yes	Yes			Yes
- Variable: big_tables			Yes		Yes
bind-address	Yes	Yes
Binlog_cache_disk_use				Yes	No
binlog_cache_size	Yes	Yes	Yes		Yes
Binlog_cache_use				Yes	No
binlog-do-db	Yes	Yes
binlog-ignore-db	Yes	Yes
bootstrap	Yes	Yes
bulk_insert_buffer_size	Yes	Yes	Yes		Yes
Bytes_received				Yes	No
Bytes_sent				Yes	No
character_set	Yes		Yes		No
character_set_client			Yes		Yes
character-set-client-handshake	Yes
character_set_connection			Yes		Yes
character_set_database			Yes		Yes
character_set_results			Yes		Yes
character-sets-dir	Yes	Yes			No
- Variable: character_sets_dir			Yes		No
character-set-server	Yes	Yes			Yes
- Variable: character_set_server			Yes		Yes
character_set_system			Yes		No
chroot	Yes	Yes
collation_connection			Yes		Yes
collation_database			Yes		Yes
collation-server	Yes	Yes			Yes
- Variable: collation_server			Yes		Yes
Com_admin_commands				Yes	No
Com_alter_db				Yes	No
Com_alter_table				Yes	No
Com_analyze				Yes	No
Com_backup_table				Yes	No
Com_begin				Yes	No
Com_change_db				Yes	No
Com_change_master				Yes	No
Com_check				Yes	No
Com_checksum				Yes	No
Com_commit				Yes	No
Com_create_db				Yes	No
Com_create_index				Yes	No
Com_create_table				Yes	No
Com_dealloc_sql				Yes	No
Com_delete				Yes	No
Com_delete_multi				Yes	No
Com_do				Yes	No
Com_drop_db				Yes	No
Com_drop_index				Yes	No
Com_drop_table				Yes	No
Com_drop_user				Yes	No
Com_execute_sql				Yes	No
Com_flush				Yes	No
Com_grant				Yes	No
Com_ha_close				Yes	No
Com_ha_open				Yes	No
Com_ha_read				Yes	No
Com_help				Yes	No
Com_insert				Yes	No
Com_insert_select				Yes	No
Com_kill				Yes	No
Com_load				Yes	No
Com_load_master_data				Yes	No
Com_load_master_table				Yes	No
Com_lock_tables				Yes	No
Com_optimize				Yes	No
Com_preload_keys				Yes	No
Com_prepare_sql				Yes	No
Com_rename_table				Yes	No
Com_repair				Yes	No
Com_replace				Yes	No
Com_replace_select				Yes	No
Com_reset				Yes	No
Com_restore_table				Yes	No
Com_revoke				Yes	No
Com_revoke_all				Yes	No
Com_rollback				Yes	No
Com_savepoint				Yes	No
Com_select				Yes	No
Com_set_option				Yes	No
Com_show_binlog_events				Yes	No
Com_show_binlogs				Yes	No
Com_show_charsets				Yes	No
Com_show_collations				Yes	No
Com_show_column_types				Yes	No
Com_show_create_db				Yes	No
Com_show_create_event				Yes	No
Com_show_create_table				Yes	No
Com_show_databases				Yes	No
Com_show_engine_logs				Yes	No
Com_show_engine_mutex				Yes	No
Com_show_engine_status				Yes	No
Com_show_errors				Yes	No
Com_show_fields				Yes	No
Com_show_grants				Yes	No
Com_show_innodb_status				Yes	No
Com_show_keys				Yes	No
Com_show_logs				Yes	No
Com_show_master_status				Yes	No
Com_show_ndb_status				Yes	No
Com_show_new_master				Yes	No
Com_show_open_tables				Yes	No
Com_show_privileges				Yes	No
Com_show_processlist				Yes	No
Com_show_slave_hosts				Yes	No
Com_show_slave_status				Yes	No
Com_show_status				Yes	No
Com_show_storage_engines				Yes	No
Com_show_tables				Yes	No
Com_show_variables				Yes	No
Com_show_warnings				Yes	No
Com_slave_start				Yes	No
Com_slave_stop				Yes	No
Com_stmt_close				Yes	No
Com_stmt_execute				Yes	No
Com_stmt_fetch				Yes	No
Com_stmt_prepare				Yes	No
Com_stmt_reset				Yes	No
Com_stmt_send_long_data				Yes	No
Com_truncate				Yes	No
Com_unlock_tables				Yes	No
Com_update				Yes	No
Com_update_multi				Yes	No
concurrent_insert	Yes	Yes	Yes		Yes
connect_timeout	Yes	Yes	Yes		Yes
console	Yes	Yes
core-file	Yes	Yes
crash_binlog_innodb	Yes	Yes
Created_tmp_disk_tables				Yes	No
Created_tmp_files				Yes	No
Created_tmp_tables				Yes	No
datadir	Yes	Yes	Yes		No
date_format			Yes		Yes
datetime_format	Yes	Yes	Yes		Yes
debug	Yes	Yes	Yes		Yes
default-character-set	Yes	Yes
default-collation	Yes		Yes		No
defaults-extra-file	Yes
defaults-file	Yes
default-storage-engine	Yes	Yes
default-table-type	Yes	Yes
default-time-zone	Yes	Yes
default_week_format	Yes	Yes	Yes		Yes
delayed_insert_limit	Yes	Yes	Yes		Yes
delayed_insert_timeout	Yes	Yes	Yes		Yes
delayed_queue_size	Yes	Yes	Yes		Yes
delay-key-write	Yes	Yes			Yes
- Variable: delay_key_write			Yes		Yes
des-key-file	Yes	Yes
disconnect-slave-event-count	Yes	Yes
enable-locking	Yes
enable-pstack	Yes	Yes
error_count			Yes		No
exit-info	Yes	Yes
expire_logs_days	Yes	Yes	Yes		Yes
external-locking	Yes	Yes
- Variable: external_locking
flush	Yes	Yes	Yes		Yes
flush_time	Yes	Yes	Yes		Yes
foreign_key_checks			Yes		Yes
ft_boolean_syntax	Yes	Yes	Yes		Yes
ft_max_word_len	Yes	Yes	Yes		No
ft_min_word_len	Yes	Yes	Yes		No
ft_query_expansion_limit	Yes	Yes	Yes		No
ft_stopword_file	Yes	Yes	Yes		No
gdb	Yes	Yes
group_concat_max_len	Yes	Yes	Yes		Yes
Handler_commit				Yes	No
Handler_delete				Yes	No
Handler_discover				Yes	No
Handler_read_first				Yes	No
Handler_read_key				Yes	No
Handler_read_next				Yes	No
Handler_read_prev				Yes	No
Handler_read_rnd				Yes	No
Handler_read_rnd_next				Yes	No
Handler_rollback				Yes	No
Handler_update				Yes	No
Handler_write				Yes	No
have_archive			Yes		No
have_bdb			Yes		No
have_blackhole_engine			Yes		No
have_compress			Yes		No
have_crypt			Yes		No
have_csv			Yes		No
have_example_engine			Yes		No
have_geometry			Yes		No
have_innodb			Yes		No
have_isam			Yes		No
have_merge_engine			Yes		No
have_ndbcluster			Yes		No
have_openssl			Yes		No
have_query_cache			Yes		No
have_raid			Yes		No
have_rtree_keys			Yes		No
have_symlink			Yes		No
help	Yes
identity			Yes		Yes
init_connect	Yes	Yes	Yes		Yes
init-file	Yes	Yes			No
- Variable: init_file			Yes		No
init_slave	Yes	Yes	Yes		Yes
innodb	Yes	Yes
innodb_additional_mem_pool_size	Yes	Yes	Yes		No
innodb_autoextend_increment	Yes	Yes	Yes		Yes
innodb_buffer_pool_awe_mem_mb	Yes	Yes	Yes		No
innodb_buffer_pool_size	Yes	Yes	Yes		No
innodb_data_file_path	Yes	Yes	Yes		No
innodb_data_home_dir	Yes	Yes	Yes		No
innodb_fast_shutdown	Yes	Yes	Yes		Yes
innodb_file_io_threads	Yes	Yes	Yes		No
innodb_file_per_table	Yes	Yes	Yes		No
innodb_flush_log_at_trx_commit	Yes	Yes	Yes		Yes
innodb_flush_method	Yes	Yes	Yes		No
innodb_force_recovery	Yes	Yes	Yes		No
innodb_locks_unsafe_for_binlog	Yes	Yes	Yes		No
innodb_lock_wait_timeout	Yes	Yes	Yes		No
innodb_log_arch_dir	Yes	Yes	Yes		No
innodb_log_archive	Yes	Yes	Yes		No
innodb_log_buffer_size	Yes	Yes	Yes		No
innodb_log_files_in_group	Yes	Yes	Yes		No
innodb_log_file_size	Yes	Yes	Yes		No
innodb_log_group_home_dir	Yes	Yes	Yes		No
innodb_max_dirty_pages_pct	Yes	Yes	Yes		Yes
innodb_max_purge_lag	Yes	Yes	Yes		Yes
innodb_mirrored_log_groups	Yes	Yes	Yes		No
innodb_open_files	Yes	Yes	Yes		No
innodb_status_file	Yes	Yes	Yes		No
innodb_table_locks	Yes	Yes	Yes		Yes
innodb_thread_concurrency	Yes	Yes	Yes		Yes
insert_id			Yes		Yes
interactive_timeout	Yes	Yes	Yes		Yes
isam	Yes	Yes	Yes		No
join_buffer_size	Yes	Yes	Yes		Yes
Key_blocks_not_flushed				Yes	No
Key_blocks_unused				Yes	No
Key_blocks_used				Yes	No
key_buffer_size	Yes	Yes	Yes		Yes
key_cache_age_threshold	Yes	Yes	Yes		Yes
key_cache_block_size	Yes	Yes	Yes		Yes
key_cache_division_limit	Yes	Yes	Yes		Yes
Key_read_requests				Yes	No
Key_reads				Yes	No
Key_write_requests				Yes	No
Key_writes				Yes	No
language	Yes	Yes	Yes		No
last_insert_id			Yes		Yes
lc_time_names			Yes		Yes
license			Yes		No
local_infile			Yes		Yes
local-infile	Yes	Yes
locked_in_memory			Yes		No
log	Yes	Yes	Yes		No
log_bin			Yes		No
log-bin	Yes	Yes
log-bin-index	Yes	Yes
log-error	Yes	Yes			No
- Variable: log_error			Yes		No
log-isam	Yes	Yes
log-long-format	Yes	Yes
log-queries-not-using-indexes	Yes	Yes			Yes
- Variable: log_queries_not_using_indexes			Yes		Yes
log-short-format	Yes	Yes
log-slave-updates	Yes	Yes			No
- Variable: log_slave_updates			Yes		No
log-slow-admin-statements	Yes	Yes
log-slow-queries	Yes	Yes			No
- Variable: log_slow_queries			Yes		No
log-update	Yes
log-warnings	Yes	Yes			Yes
- Variable: log_warnings			Yes		Yes
long_query_time	Yes	Yes	Yes		Yes
lower_case_file_system	Yes	Yes	Yes		No
lower_case_table_names	Yes	Yes	Yes		No
low-priority-updates	Yes	Yes			Yes
- Variable: low_priority_updates			Yes		Yes
master-connect-retry	Yes	Yes
master-host	Yes	Yes
master-info-file	Yes	Yes
master-password	Yes	Yes
master-port	Yes	Yes
master-retry-count	Yes	Yes
master-ssl	Yes	Yes
master-ssl-ca	Yes	Yes
master-ssl-capath	Yes	Yes
master-ssl-cert	Yes	Yes
master-ssl-cipher	Yes	Yes
master-ssl-key	Yes	Yes
master-user	Yes	Yes
max_allowed_packet	Yes	Yes	Yes		Yes
max_binlog_cache_size	Yes	Yes	Yes		Yes
max-binlog-dump-events	Yes	Yes
max_binlog_size	Yes	Yes	Yes		Yes
max_connect_errors	Yes	Yes	Yes		Yes
max_connections	Yes	Yes	Yes		Yes
max_delayed_threads	Yes	Yes	Yes		Yes
max_error_count	Yes	Yes	Yes		Yes
max_heap_table_size	Yes	Yes	Yes		Yes
max_insert_delayed_threads			Yes		Yes
max_join_size	Yes	Yes	Yes		Yes
max_length_for_sort_data	Yes	Yes	Yes		Yes
max_prepared_stmt_count	Yes	Yes	Yes		Yes
max_relay_log_size	Yes	Yes	Yes		Yes
max_seeks_for_key	Yes	Yes	Yes		Yes
max_sort_length	Yes	Yes	Yes		Yes
max_tmp_tables	Yes	Yes	Yes		Yes
Max_used_connections				Yes	No
max_user_connections	Yes	Yes	Yes		Yes
max_write_lock_count	Yes	Yes	Yes		Yes
memlock	Yes	Yes	Yes		No
merge	Yes	Yes
myisam_block_size	Yes	Yes	Yes		Yes
myisam_data_pointer_size	Yes	Yes	Yes		Yes
myisam_max_extra_sort_file_size	Yes	Yes	Yes		No
myisam_max_sort_file_size	Yes	Yes	Yes		Yes
myisam-recover	Yes	Yes
myisam_recover_options			Yes		No
myisam_repair_threads	Yes	Yes	Yes		Yes
myisam_sort_buffer_size	Yes	Yes	Yes		Yes
myisam_stats_method	Yes	Yes	Yes		Yes
named_pipe			Yes		No
ndb_autoincrement_prefetch_sz	Yes	Yes	Yes		Yes
ndb_cache_check_time	Yes	Yes	Yes		Yes
ndbcluster	Yes	Yes	Yes		Yes
ndb_force_send	Yes	Yes	Yes		Yes
ndb_index_stat_cache_entries	Yes	Yes
ndb_index_stat_enable	Yes	Yes
ndb_index_stat_update_freq	Yes	Yes
ndb_optimized_node_selection	Yes	Yes
ndb_report_thresh_binlog_epoch_slip	Yes	Yes
ndb_report_thresh_binlog_mem_usage	Yes	Yes
ndb_use_exact_count			Yes		Yes
ndb_use_transactions	Yes	Yes
net_buffer_length	Yes	Yes	Yes		Yes
net_read_timeout	Yes	Yes	Yes		Yes
net_retry_count	Yes	Yes	Yes		Yes
net_write_timeout	Yes	Yes	Yes		Yes
new	Yes	Yes	Yes		Yes
no-defaults	Yes
Not_flushed_delayed_rows				Yes	No
old-passwords	Yes	Yes			Yes
- Variable: old_passwords			Yes		Yes
old-protocol	Yes	Yes
Opened_tables				Yes	No
Open_files				Yes	No
open-files-limit	Yes	Yes			No
- Variable: open_files_limit			Yes		No
Open_streams				Yes	No
Open_tables				Yes	No
pid-file	Yes	Yes			No
- Variable: pid_file			Yes		No
port	Yes	Yes	Yes		No
preload_buffer_size	Yes	Yes	Yes		Yes
prepared_stmt_count			Yes	Yes	No
print-defaults	Yes
protocol_version			Yes		No
Qcache_free_blocks				Yes	No
Qcache_free_memory				Yes	No
Qcache_hits				Yes	No
Qcache_inserts				Yes	No
Qcache_lowmem_prunes				Yes	No
Qcache_not_cached				Yes	No
Qcache_queries_in_cache				Yes	No
Qcache_total_blocks				Yes	No
query_alloc_block_size	Yes	Yes	Yes		Yes
query_cache_limit	Yes	Yes	Yes		Yes
query_cache_min_res_unit	Yes	Yes	Yes		Yes
query_cache_size	Yes	Yes	Yes		Yes
query_cache_type	Yes	Yes	Yes		Yes
query_cache_wlock_invalidate	Yes	Yes	Yes		Yes
query_prealloc_size	Yes	Yes	Yes		Yes
Questions				Yes	No
range_alloc_block_size	Yes	Yes	Yes		Yes
read_buffer_size	Yes	Yes	Yes		Yes
read_only	Yes	Yes	Yes		Yes
read_rnd_buffer_size	Yes	Yes	Yes		Yes
relay-log	Yes	Yes
relay-log-index	Yes	Yes
relay-log-info-file	Yes	Yes
relay_log_purge	Yes	Yes	Yes		Yes
relay_log_space_limit	Yes	Yes	Yes		No
replicate-do-db	Yes	Yes
replicate-do-table	Yes	Yes
replicate-ignore-db	Yes	Yes
replicate-ignore-table	Yes	Yes
replicate-rewrite-db	Yes	Yes
replicate-same-server-id	Yes	Yes
replicate-wild-do-table	Yes	Yes
replicate-wild-ignore-table	Yes	Yes
report-host	Yes	Yes			No
- Variable: report_host			Yes		No
report-password	Yes	Yes			No
- Variable: report_password			Yes		No
report-port	Yes	Yes			No
- Variable: report_port			Yes		No
report-user	Yes	Yes			No
- Variable: report_user			Yes		No
rpl_recovery_rank			Yes		Yes
safemalloc-mem-limit	Yes	Yes
safe-mode	Yes	Yes
safe-show-database	Yes	Yes			Yes
- Variable: safe_show_database			Yes		Yes
safe-user-create	Yes	Yes
secure-auth	Yes	Yes			Yes
- Variable: secure_auth			Yes		Yes
Select_full_join				Yes	No
Select_full_range_join				Yes	No
Select_range				Yes	No
Select_range_check				Yes	No
Select_scan				Yes	No
server-id	Yes	Yes			Yes
- Variable: server_id			Yes		Yes
set-variable	Yes	Yes
shared_memory			Yes		No
shared_memory_base_name			Yes		No
show-slave-auth-info	Yes	Yes
skip-bdb	Yes	Yes
skip-character-set-client-handshake	Yes	Yes
skip-concurrent-insert	Yes	Yes
- Variable: concurrent_insert
skip-external-locking	Yes	Yes			No
- Variable: skip_external_locking			Yes		No
skip-grant-tables	Yes	Yes
skip-host-cache	Yes	Yes
skip-innodb	Yes	Yes
skip-isam	Yes	Yes	Yes		No
skip-locking	Yes	Yes
skip-log-warnings	Yes
skip-name-resolve	Yes	Yes
skip-networking	Yes	Yes			No
- Variable: skip_networking			Yes		No
skip-new	Yes	Yes
skip-safemalloc	Yes	Yes
skip-show-database	Yes	Yes			No
- Variable: skip_show_database			Yes		No
skip-slave-start	Yes	Yes
skip-ssl	Yes	Yes
skip-stack-trace	Yes	Yes
skip-symbolic-links	Yes
skip-symlink	Yes	Yes
skip-sync-bdb-logs	Yes	Yes	Yes		No
skip-thread-priority	Yes	Yes
slave_compressed_protocol	Yes	Yes	Yes		Yes
slave-load-tmpdir	Yes	Yes			No
- Variable: slave_load_tmpdir			Yes		No
slave-net-timeout	Yes	Yes			Yes
- Variable: slave_net_timeout			Yes		Yes
Slave_open_temp_tables				Yes	No
slave-skip-errors	Yes	Yes			No
- Variable: slave_skip_errors			Yes		No
slave_transaction_retries	Yes	Yes	Yes		Yes
Slow_launch_threads				Yes	No
slow_launch_time	Yes	Yes	Yes		Yes
Slow_queries				Yes	No
socket	Yes	Yes	Yes		No
sort_buffer_size	Yes	Yes	Yes		Yes
Sort_merge_passes				Yes	No
Sort_range				Yes	No
Sort_rows				Yes	No
Sort_scan				Yes	No
sporadic-binlog-dump-fail	Yes	Yes
sql_auto_is_null			Yes		Yes
sql_big_selects			Yes		Yes
sql_big_tables			Yes		Yes
sql-bin-update-same	Yes	Yes
sql_buffer_result			Yes		Yes
sql_log_bin			Yes		Yes
sql_log_off			Yes		Yes
sql_log_update			Yes		Yes
sql_low_priority_updates			Yes		Yes
sql_max_join_size			Yes		Yes
sql-mode	Yes	Yes			Yes
- Variable: sql_mode			Yes		Yes
sql_notes			Yes		Yes
sql_quote_show_create			Yes		Yes
sql_safe_updates			Yes		Yes
sql_select_limit	Yes	Yes	Yes		Yes
sql_slave_skip_counter			Yes		Yes
sql_warnings			Yes		Yes
ssl	Yes	Yes
ssl-ca	Yes	Yes			No
- Variable: ssl_ca			Yes		No
ssl-capath	Yes	Yes			No
- Variable: ssl_capath			Yes		No
ssl-cert	Yes	Yes			No
- Variable: ssl_cert			Yes		No
ssl-cipher	Yes	Yes			No
- Variable: ssl_cipher			Yes		No
ssl-key	Yes	Yes			No
- Variable: ssl_key			Yes		No
standalone	Yes	Yes
storage_engine			Yes		Yes
symbolic-links	Yes	Yes
sync-bdb-logs	Yes	Yes	Yes		No
sync-binlog	Yes	Yes			Yes
- Variable: sync_binlog			Yes		Yes
sync-frm	Yes	Yes			Yes
- Variable: sync_frm			Yes		Yes
system_time_zone			Yes		No
table_cache	Yes	Yes	Yes		Yes
Table_locks_immediate				Yes	No
Table_locks_waited				Yes	No
table_type			Yes		Yes
temp-pool	Yes	Yes
thread_cache_size	Yes	Yes	Yes		Yes
thread_concurrency	Yes	Yes	Yes		No
Threads_cached				Yes	No
Threads_connected				Yes	No
Threads_created				Yes	No
Threads_running				Yes	No
thread_stack	Yes	Yes	Yes		No
time_format	Yes	Yes	Yes		Yes
timestamp			Yes		Yes
time_zone	Yes		Yes		Yes
tmpdir	Yes	Yes	Yes		No
tmp_table_size	Yes	Yes	Yes		Yes
transaction_alloc_block_size	Yes	Yes	Yes		Yes
transaction-isolation	Yes	Yes
transaction_prealloc_size	Yes	Yes	Yes		Yes
tx_isolation			Yes		Yes
unique_checks			Yes		Yes
Uptime				Yes	No
user	Yes	Yes
verbose	Yes
version	Yes		Yes		No
version_comment			Yes		No
version_compile_machine			Yes		No
version_compile_os			Yes		No
wait_timeout	Yes	Yes	Yes		Yes
warning_count			Yes		No
warnings	Yes	Yes

5.1.2. Command Options

When you start the mysqld server, you can specify program options using any of the methods described in Section 4.2.3, “Specifying Program Options”. The most common methods are to provide options in an option file or on the command line. However, in most cases it is desirable to make sure that the server uses the same options each time it runs. The best way to ensure this is to list them in an option file. See Section 4.2.3.2, “Using Option Files”.

MySQL Enterprise For expert advice on setting command options, subscribe to the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

mysqld reads options from the [mysqld] and [server] groups. mysqld_safe reads options from the [mysqld], [server], [mysqld_safe], and [safe_mysqld] groups. mysql.server reads options from the [mysqld] and [mysql.server] groups.

An embedded MySQL server usually reads options from the [server], [embedded], and [xxxxx_SERVER] groups, where xxxxx is the name of the application into which the server is embedded.

mysqld accepts many command options. For a list, execute mysqld --help. Before MySQL 4.1.1, --help prints the full help message. As of 4.1.1, it prints a brief message; to see the full list, use mysqld --verbose --help.

The following list shows some of the most common server options. Additional options are described in other sections:

Options that affect security: See Section 5.4.3, “Security-Related mysqld Options”.
SSL-related options: See Section 5.6.7.3, “SSL Command Options”.
Binary log control options: See Section 5.3.4, “The Binary Log”.
Replication-related options: See Section 14.8, “Replication Startup Options”.
Options specific to particular storage engines: See Section 13.1.1, “MyISAM Startup Options”, Section 13.5.3, “BDB Startup Options”, Section 13.2.5, “InnoDB Startup Options and System Variables”, and Section 15.4.2, “MySQL Cluster-Related Command Options for mysqld”.

You can also set the values of server system variables by using variable names as options, as described at the end of this section.

--help, -?
Display a short help message and exit. Before MySQL 4.1.1, --help displays the full help message. As of 4.1.1, it displays an abbreviated message only. Use both the --verbose and --help options to see the full message.
--abort-slave-event-count
When this option is set to some positive integer value other than 0 (the default) it affects replication behavior as follows: After the slave SQL thread has started, value log events are allowed to be executed; after that, the slave SQL thread does not receive any more events, just as if the network connection from the master were cut. The slave thread continues to run, and the output from SHOW SLAVE STATUS displays Yes in both the Slave_IO_Running and the Slave_SQL_Running columns, but no further events are read from the relay log.
This option is used internally by the MySQL test suite for replication testing and debugging. It is not intended for use in a production setting.
--allow-suspicious-udfs
This option controls whether user-defined functions that have only an xxx symbol for the main function can be loaded. By default, the option is off and only UDFs that have at least one auxiliary symbol can be loaded; this prevents attempts at loading functions from shared object files other than those containing legitimate UDFs. This option was added in MySQL 4.0.24, and 4.1.10a. See Section 19.2.4.6, “User-Defined Function Security Precautions”.
--ansi
Use standard (ANSI) SQL syntax instead of MySQL syntax. For more precise control over the server SQL mode, use the --sql-mode option instead. See Section 1.8.3, “Running MySQL in ANSI Mode”, and Section 5.1.7, “SQL Modes”.
--basedir=path, -b path
The path to the MySQL installation directory. All paths are usually resolved relative to this directory.
--big-tables
Allow large result sets by saving all temporary sets in files. This option prevents most “table full” errors, but also slows down queries for which in-memory tables would suffice. Since MySQL 3.23.2, the server is able to handle large result sets automatically by using memory for small temporary tables and switching to disk tables where necessary.
--bind-address=IP
The IP address to bind to. Only one address can be selected. If this option is specified multiple times, the last address given is used.
--bootstrap
This option is used by the mysql_install_db script to create the MySQL privilege tables without having to start a full MySQL server.
--character-sets-dir=path
The directory where character sets are installed. See Section 9.2, “The Character Set Used for Data and Sorting”.
--character-set-client-handshake
Don't ignore character set information sent by the client. To ignore client information and use the default server character set, use --skip-character-set-client-handshake; this makes MySQL 4.1 and higher behave like MySQL 4.0. This option was added in MySQL 4.1.15.
--character-set-server=charset_name, -C charset_name
Use charset_name as the default server character set. See Section 9.2, “The Character Set Used for Data and Sorting”. If you use this option to specify a non-default character set, you should also use --collation-server to specify the collation. This option is available as of MySQL 4.1.3.
--chroot=path, -r path
Put the mysqld server in a closed environment during startup by using the chroot() system call. This is a recommended security measure as of MySQL 4.0. (MySQL 3.23 is not able to provide a chroot() jail that is 100% closed.) Note that use of this option somewhat limits LOAD DATA INFILE and SELECT ... INTO OUTFILE.
--collation-server=collation_name
Use collation_name as the default server collation. This option is available as of MySQL 4.1.3. See Section 9.2, “The Character Set Used for Data and Sorting”.
--console
(Windows only.) Write error log messages to stderr and stdout even if --log-error is specified. mysqld does not close the console window if this option is used.
--core-file
Write a core file if mysqld dies. For some systems, you must also specify the --core-file-size option to mysqld_safe. See Section 4.3.2, “mysqld_safe — MySQL Server Startup Script”. Note that on some systems, such as Solaris, you do not get a core file if you are also using the --user option.
--datadir=path, -h path
The path to the data directory.
--debug[=debug_options], -# [debug_options]
If MySQL is configured with --with-debug, you can use this option to get a trace file of what mysqld is doing. The debug_options string often is 'd:t:o,file_name'. The default is 'd:t:i:o,mysqld.trace'. See MySQL Internals: Porting.
--default-character-set=charset_name, -C charset_name
Use charset_name as the default character set. This option is deprecated in favor of --character-set-server as of MySQL 4.1.3. See Section 9.2, “The Character Set Used for Data and Sorting”.
--default-collation=collation_name
Use collation_name as the default collation. This option is deprecated in favor of --collation-server as of MySQL 4.1.3. See Section 9.2, “The Character Set Used for Data and Sorting”.
--default-storage-engine=type
This option is a synonym for --default-table-type. It is available as of MySQL 4.1.2.
--default-table-type=type
Set the default table type (storage engine) for tables. See Chapter 13, Storage Engines.
--default-time-zone=timezone
Set the default server time zone. This option sets the global time_zone system variable. If this option is not given, the default time zone is the same as the system time zone (given by the value of the system_time_zone system variable. This option is available as of MySQL 4.1.3.
--delay-key-write[={OFF|ON|ALL}]
Specify how to use delayed key writes. Delayed key writing causes key buffers not to be flushed between writes for MyISAM tables. OFF disables delayed key writes. ON enables delayed key writes for those tables that were created with the DELAY_KEY_WRITE option. ALL delays key writes for all MyISAM tables. Available as of MySQL 4.0.3. See Section 7.5.2, “Tuning Server Parameters”, and Section 13.1.1, “MyISAM Startup Options”.
Note
If you set this variable to ALL, you should not use MyISAM tables from within another program (such as another MySQL server or myisamchk) when the tables are in use. Doing so leads to index corruption.
--delay-key-write-for-all-tables
Old form of --delay-key-write=ALL for use prior to MySQL 4.0.3. As of 4.0.3, use --delay-key-write instead.
--des-key-file=file_name
Read the default DES keys from this file. These keys are used by the DES_ENCRYPT() and DES_DECRYPT() functions.
--disconnect-slave-event-count
This option is used internally by the MySQL test suite for replication testing and debugging.
--enable-named-pipe
Enable support for named pipes. This option applies only on Windows NT, 2000, XP, and 2003 systems, and can be used only with the mysqld-nt and mysqld-max-nt servers that support named-pipe connections.
--enable-pstack
Print a symbolic stack trace on failure.
--exit-info[=flags], -T [flags]
This is a bit mask of different flags that you can use for debugging the mysqld server. Do not use this option unless you know exactly what it does!
--external-locking
Enable external locking (system locking), which is disabled by default as of MySQL 4.0. Note that if you use this option on a system on which lockd does not fully work (such as Linux), it is easy for mysqld to deadlock. This option was named --enable-locking before MySQL 4.0.3.
For more information about external locking, including conditions under which it can and cannot be used, see Section 7.3.4, “External Locking”.
--flush
Flush (synchronize) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section A.1.4.2, “What to Do If MySQL Keeps Crashing”.
--gdb
Install an interrupt handler for SIGINT (needed to stop mysqld with ^C to set breakpoints) and disable stack tracing and core file handling. See MySQL Internals: Porting. This option was added in MySQL 4.0.14.
--init-file=file_name
Read SQL statements from this file at startup. Each statement must be on a single line and should not include comments.
--innodb-safe-binlog
Adds consistency guarantees between the content of InnoDB tables and the binary log. See Section 5.3.4, “The Binary Log”.
--innodb-xxx
The InnoDB options are listed in Section 13.2.5, “InnoDB Startup Options and System Variables”.
--language=lang_name, -L lang_name
Return client error messages in the given language. lang_name can be given as the language name or as the full pathname to the directory where the language files are installed. See Section 9.3, “Setting the Error Message Language”.
--log[=file_name], -l [file_name]
Log connections and SQL statements received from clients to this file. See Section 5.3.2, “The General Query Log”. If you omit the filename, MySQL uses host_name.log as the filename.
--log-bin[=base_name]
Enable binary logging. The server logs all statements that change data to the binary log, which is used for backup and replication. See Section 5.3.4, “The Binary Log”.
The option value, if given, is the basename for the log sequence. The server creates binary log files in sequence by adding a numeric suffix to the basename. It is recommended that you specify a basename (see Section A.1.8.4, “Open Issues in MySQL”, for the reason). Otherwise, MySQL uses host_name-bin as the basename.
--log-bin-index[=file_name]
The index file for binary log filenames. See Section 5.3.4, “The Binary Log”. If you omit the filename, and if you didn't specify one with --log-bin, MySQL uses host_name-bin.index as the filename.
--log-error[=file_name]
Log errors and startup messages to this file. See Section 5.3.1, “The Error Log”. If you omit the filename, MySQL uses host_name.err. If the filename has no extension, the server adds an extension of .err.
--log-isam[=file_name]
Log all ISAM/MyISAM changes to this file (used only when debugging ISAM/MyISAM).
--log-long-format
Log extra information to the update log, binary update log, and slow query log, if they have been activated. For example, the username and timestamp are logged for queries. Before MySQL 4.1, if you are using --log-slow-queries and --log-long-format, queries that are not using indexes also are logged to the slow query log. --log-long-format is deprecated as of MySQL version 4.1, when --log-short-format was introduced. (Long log format is the default setting since version 4.1.) Also note that starting with MySQL 4.1, the --log-queries-not-using-indexes option is available for the purpose of logging queries that do not use indexes to the slow query log.
--log-queries-not-using-indexes
If you are using this option with --log-slow-queries, queries that do not use indexes also are logged to the slow query log. This option is available as of MySQL 4.1. See Section 5.3.5, “The Slow Query Log”.
--log-short-format
Log less information to the update log, binary update log, and slow query log, if they have been activated. For example, the username and timestamp are not logged for queries. This option was introduced in MySQL 4.1.
--log-slow-admin-statements
Log slow administrative statements such as OPTIMIZE TABLE, ANALYZE TABLE, and ALTER TABLE to the slow query log.
This option was added in MySQL 4.1.13. (It is unnecessary in MySQL 4.0 because slow administrative statements are logged by default.)
--log-slow-queries[=file_name]
Log all queries that have taken more than long_query_time seconds to execute to this file. See Section 5.3.5, “The Slow Query Log”. Note that the default for the amount of information logged has changed in MySQL 4.1. See the --log-long-format and --log-short-format options for details.
--log-update[=file_name]
Log updates to fileN where N is a unique number if not given. See Section 5.3.3, “The Update Log”. The update log is now deprecated; you should use the binary log instead (--log-bin). See Section 5.3.4, “The Binary Log”.
--log-warnings[=level], -W [level]
Print out warnings such as Aborted connection... to the error log. Enabling this option is recommended, for example, if you use replication (you get more information about what is happening, such as messages about network failures and reconnections). This option is enabled by default as of MySQL 4.0.19 and 4.1.2; to disable it, use --log-warnings=0. As of MySQL 4.0.21 and 4.1.3, a level argument can be given. If omitted, the default level is 1. If the value is greater than 1, aborted connections are written to the error log. See Section A.1.2.11, “Communication Errors and Aborted Connections”.
Before MySQL 4.0.21 and 4.1.3, this is a boolean option, not an integer-valued option. Before 4.0, this option was named --warnings.
--low-priority-updates
Give table-modifying operations (INSERT, REPLACE, DELETE, UPDATE) lower priority than selects. This can also be done via {INSERT | REPLACE | DELETE | UPDATE} LOW_PRIORITY ... to lower the priority of only one query, or by SET LOW_PRIORITY_UPDATES=1 to change the priority in one thread. This affects only storage engines that use only table-level locking (MyISAM, MEMORY, MERGE). See Section 7.3.2, “Table Locking Issues”.
--max-binlog-dump-events
This option is used internally by the MySQL test suite for replication testing and debugging.
--memlock
Lock the mysqld process in memory. This option might help if you have a problem where the operating system is causing mysqld to swap to disk.
--memlock works on systems that support the mlockall() system call; this includes Solaris as well as most Linux distributions that use a 2.4 or newer kernel. On Linux systems, you can tell whether or not mlockall() (and thus this option) is supported by checking to see whether or not it is defined in the system mman.h file, like this:
```
shell> grep mlockall /usr/include/sys/mman.h 
```
If mlockall() is supported, you should see in the output of the previous command something like the following:
```
extern int mlockall (int __flags) __THROW;
```
Important
Using this option requires that you run the server as root, which, for reasons of security, is normally not a good idea. See Section 5.4.5, “How to Run MySQL as a Normal User”.
You must not try to use this option on a system that does not support the mlockall() system call; if you do so, mysqld will very likely crash as soon as you try to start it.

--myisam-recover[=option[,option]...]]

Set the MyISAM storage engine recovery mode. The option value is any combination of the values of DEFAULT, BACKUP, FORCE, or QUICK. If you specify multiple values, separate them by commas. You can also use a value of "" to disable this option. If this option is used, each time mysqld opens a MyISAM table, it checks whether the table is marked as crashed or wasn't closed properly. (The last option works only if you are running with external locking disabled.) If this is the case, mysqld runs a check on the table. If the table was corrupted, mysqld attempts to repair it.

The following options affect how the repair works:

Option	Description
`DEFAULT`	Recovery without backup, forcing, or quick checking.
`BACKUP`	If the data file was changed during recovery, save a backup of the `tbl_name.MYD` file as `tbl_name-datetime.BAK`.
`FORCE`	Run recovery even if we would lose more than one row from the `.MYD` file.
`QUICK`	do not check the rows in the table if there are not any delete blocks.

Before the server automatically repairs a table, it writes a note about the repair to the error log. If you want to be able to recover from most problems without user intervention, you should use the options BACKUP,FORCE. This forces a repair of a table even if some rows would be deleted, but it keeps the old data file as a backup so that you can later examine what happened.

See Section 13.1.1, “MyISAM Startup Options”.

This option is available as of MySQL 3.23.25.

--new
The --new option can be used to make the server behave as 4.1 in certain respects, easing a 4.0 to 4.1 upgrade:
- Hexadecimal strings such as 0xFF are treated as strings by default rather than as numbers. (Works in 4.0.12 and up.)
- TIMESTAMP is returned as a string with the format 'YYYY-MM-DD HH:MM:SS'. (Works in 4.0.13 and up.) See Chapter 10, Data Types.
This option can be used to help you see how your applications behave in MySQL 4.1, without actually upgrading to 4.1.
--old-passwords
Force the server to generate short (pre-4.1) password hashes for new passwords. This is useful for compatibility when the server must support older client programs. See Section 5.5.8, “Password Hashing as of MySQL 4.1”.
--old-protocol, -o
Use the 3.20 protocol for compatibility with some very old clients. This option was removed in MySQL 4.1.1.
--one-thread
Only use one thread (for debugging under Linux). This option is available only if the server is built with debugging enabled. See MySQL Internals: Porting.
--open-files-limit=count
Changes the number of file descriptors available to mysqld. You should try increasing the value of this option if mysqld gives you the error Too many open files. mysqld uses the option value to reserve descriptors with setrlimit(). If the requested number of file descriptors cannot be allocated, mysqld writes a warning to the error log.
mysqld may attempt to allocate more than the requested number of descriptors (if they are available), using the values of max_connections and table_cache to estimate whether more descriptors will be needed.
--pid-file=path
The pathname of the process ID file. This file is used by other programs such as mysqld_safe to determine the server's process ID.
--port=port_num, -P port_num
The port number to use when listening for TCP/IP connections. The port number must be 1024 or higher unless the server is started by the root system user.
--safe-mode
Skip some optimization stages.
--safe-show-database
With this option, the SHOW DATABASES statement displays only the names of those databases for which the user has some kind of privilege. As of MySQL 4.0.2, this option is deprecated and does not do anything (it is enabled by default), because there is a SHOW DATABASES privilege that can be used to control access to database names on a per-account basis. See Section 5.5.3, “Privileges Provided by MySQL”.
--safe-user-create
If this option is enabled, a user cannot create new MySQL users by using the GRANT statement, if the user doesn't have the INSERT privilege for the mysql.user table or any column in the table.
--secure-auth
Disallow authentication by clients that attempt to use accounts that have old (pre-4.1) passwords. This option is available as of MySQL 4.1.1.
--shared-memory
Enable shared-memory connections by local clients. This option is available only on Windows. It was added in MySQL 4.1.0.
--shared-memory-base-name=name
The name of shared memory to use for shared-memory connections. This option is available only on Windows. The default name is MYSQL. The name is case sensitive. This option was added in MySQL 4.1.0.
--skip-bdb
Disable the BDB storage engine. This saves memory and might speed up some operations. Do not use this option if you require BDB tables.
--skip-concurrent-insert
Turn off the ability to select and insert at the same time on MyISAM tables. (This is to be used only if you think you have found a bug in this feature.) See Section 7.3.3, “Concurrent Inserts”.
--skip-delay-key-write
Ignore the DELAY_KEY_WRITE option for all tables. As of MySQL 4.0.3, you should use --delay-key-write=OFF instead. See Section 7.5.2, “Tuning Server Parameters”.
--skip-external-locking
Do not use external locking (system locking). For more information about external locking, including conditions under which it can and cannot be used, see Section 7.3.4, “External Locking”.
External locking has been disabled by default since MySQL 4.0.
--skip-grant-tables
This option causes the server not to use the privilege system at all, which gives anyone with access to the server unrestricted access to all databases. You can cause a running server to start using the grant tables again by executing mysqladmin flush-privileges or mysqladmin reload command from a system shell, or by issuing a MySQL FLUSH PRIVILEGES statement after connecting to the server. This option also suppresses loading of user-defined functions (UDFs).
--skip-host-cache
Do not use the internal hostname cache for faster name-to-IP resolution. Instead, query the DNS server every time a client connects. See Section 7.5.9, “How MySQL Uses DNS”.
--skip-innodb
Disable the InnoDB storage engine. This saves memory and disk space and might speed up some operations. Do not use this option if you require InnoDB tables.
--skip-isam
Disable the ISAM storage engine. As of MySQL 4.1, ISAM is disabled by default, so this option applies only if the server was configured with support for ISAM. This option was added in MySQL 4.1.1.
--skip-merge
Disable the MERGE storage engine. This option was added in MySQL 4.1.21. It can be used if the following behavior is undesirable: If a user has access to MyISAM table t, that user can create a MERGE table m that accesses t. However, if the user's privileges on t are subsequently revoked, the user can continue to access t by doing so through m.
--skip-name-resolve
Do not resolve hostnames when checking client connections. Use only IP numbers. If you use this option, all Host column values in the grant tables must be IP numbers or localhost. See Section 7.5.9, “How MySQL Uses DNS”.
--skip-networking
Do not listen for TCP/IP connections at all. All interaction with mysqld must be made via named pipes or shared memory (on Windows) or Unix socket files (on Unix). This option is highly recommended for systems where only local clients are allowed. See Section 7.5.9, “How MySQL Uses DNS”.
--skip-new
do not use new, possibly wrong routines.
--skip-symlink
This is the old form of --skip-symbolic-links, for use before MySQL 4.0.13.
--sporadic-binlog-dump-fail
This option is used internally by the MySQL test suite for replication testing and debugging.
--ssl*
Options that begin with --ssl specify whether to allow clients to connect via SSL and indicate where to find SSL keys and certificates. See Section 5.6.7.3, “SSL Command Options”.
--standalone
Available on Windows NT-based systems only; instructs the MySQL server not to run as a service.
--symbolic-links, --skip-symbolic-links
Enable or disable symbolic link support. This option has different effects on Windows and Unix:
- On Windows, enabling symbolic links allows you to establish a symbolic link to a database directory by creating a db_name.sym file that contains the path to the real directory. See Section 7.6.1.3, “Using Symbolic Links for Databases on Windows”.
- On Unix, enabling symbolic links means that you can link a MyISAM index file or data file to another directory with the INDEX DIRECTORY or DATA DIRECTORY options of the CREATE TABLE statement. If you delete or rename the table, the files that its symbolic links point to also are deleted or renamed. See Section 7.6.1.2, “Using Symbolic Links for Tables on Unix”.
This option was added in MySQL 4.0.13.
--skip-safemalloc
If MySQL is configured with --with-debug=full, all MySQL programs check for memory overruns during each memory allocation and memory freeing operation. This checking is very slow, so for the server you can avoid it when you do not need it by using the --skip-safemalloc option.
--skip-show-database
With this option, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. Without this option, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database. Note that any global privilege is considered a privilege for the database.
--skip-stack-trace
do not write stack traces. This option is useful when you are running mysqld under a debugger. On some systems, you also must use this option to get a core file. See MySQL Internals: Porting.
--skip-thread-priority
Disable using thread priorities for faster response time.
--socket=path
On Unix, this option specifies the Unix socket file to use when listening for local connections. The default value is /tmp/mysql.sock. On Windows, the option specifies the pipe name to use when listening for local connections that use a named pipe. The default value is MySQL (not case sensitive).
--sql-mode=value[,value[,value...]]
Set the SQL mode. See Section 5.1.7, “SQL Modes”. This option was added in 3.23.41.
--temp-pool
This option causes most temporary files created by the server to use a small set of names, rather than a unique name for each new file. This works around a problem in the Linux kernel dealing with creating many new files with different names. With the old behavior, Linux seems to “leak” memory, because it is being allocated to the directory entry cache rather than to the disk cache.
--transaction-isolation=level
Sets the default transaction isolation level. The level value can be READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE. See Section 12.4.6, “SET TRANSACTION Syntax”.
--tmpdir=path, -t path
The path of the directory to use for creating temporary files. It might be useful if your default /tmp directory resides on a partition that is too small to hold temporary tables. Starting from MySQL 4.1.0, this option accepts several paths that are used in round-robin fashion. Paths should be separated by colon characters (“:”) on Unix and semicolon characters (“;”) on Windows, NetWare, and OS/2. If the MySQL server is acting as a replication slave, you should not set --tmpdir to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. For more information about the storage location of temporary files, see Section A.1.4.4, “Where MySQL Stores Temporary Files”. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication fails.
--user={user_name|user_id}, -u {user_name|user_id}
Run the mysqld server as the user having the name user_name or the numeric user ID user_id. (“User” in this context refers to a system login account, not a MySQL user listed in the grant tables.)
This option is mandatory when starting mysqld as root. The server changes its user ID during its startup sequence, causing it to run as that particular user rather than as root. See Section 5.4.1, “General Security Guidelines”.
Starting from MySQL 3.23.56 and 4.0.12: To avoid a possible security hole where a user adds a --user=root option to a my.cnf file (thus causing the server to run as root), mysqld uses only the first --user option specified and produces a warning if there are multiple --user options. Options in /etc/my.cnf and $MYSQL_HOME/my.cnf are processed before command-line options, so it is recommended that you put a --user option in /etc/my.cnf and specify a value other than root. The option in /etc/my.cnf is found before any other --user options, which ensures that the server runs as a user other than root, and that a warning results if any other --user option is found.
--version, -V
Display version information and exit.

As of MySQL 4.0, you can assign a value to a server system variable by using an option of the form --var_name=value. For example, --key_buffer_size=32M sets the key_buffer_size variable to a value of 32MB.

Note that when you assign a value to a variable, MySQL might automatically correct the value to stay within a given range, or adjust the value to the closest allowable value if only certain values are allowed.

If you want to restrict the maximum value to which a variable can be set at runtime with SET, you can define this by using the --maximum-var_name=value command-line option.

It is also possible to set variables by using --set-variable=var_name=value or --var_name=value syntax. This syntax is deprecated as of MySQL 4.0.

You can change the values of most system variables for a running server with the SET statement. See Section 12.5.3, “SET Syntax”.

Section 5.1.3, “System Variables”, provides a full description for all variables, and additional information for setting them at server startup and runtime. Section 7.5.2, “Tuning Server Parameters”, includes information on optimizing the server by tuning system variables.

5.1.3. System Variables

The MySQL server maintains many system variables that indicate how it is configured. Each system variable has a default value. System variables can be set at server startup using options on the command line or in an option file. As of MySQL 4.0.3, most of them can be changed dynamically while the server is running by means of the SET statement, which enables you to modify operation of the server without having to stop and restart it. You can refer to system variable values in expressions.

There are several ways to see the names and values of system variables:

To see the values that a server will use based on its compiled-in defaults and any option files that it reads, use this command (omit --verbose before MySQL 4.1.1):
```
mysqld --verbose --help
```
To see the values that a server will use based on its compiled-in defaults, ignoring the settings in any option files, use this command (omit --verbose before MySQL 4.1.1):
```
mysqld --no-defaults --verbose --help
```
To see the current values used by a running server, use the SHOW VARIABLES statement.

This section provides a description of each system variable. Variables with no version indicated have been present since at least MySQL 3.22.

The following table lists all available system variables:

Variable Name	Variable Scope	Dynamic?
`autocommit`	`SESSION`	yes
`back_log`	`GLOBAL`	no
`basedir`	`GLOBAL`	no
`bdb_cache_size`	`GLOBAL`	no
`bdb_home`	`GLOBAL`	no
`bdb_lock_detect`	`GLOBAL`	no
`bdb_log_buffer_size`	`GLOBAL`	no
`bdb_logdir`	`GLOBAL`	no
`bdb_max_lock`	`GLOBAL`	no
`bdb_shared_data`	`GLOBAL`	no
`bdb_tmpdir`	`GLOBAL`	no
`big_tables`	`SESSION`	yes
`binlog_cache_size`	`GLOBAL`	yes
`bulk_insert_buffer_size`	`GLOBAL` \| `SESSION`	yes
`character_set`		no
`character_set_client`	`GLOBAL` \| `SESSION`	yes
`character_set_connection`	`GLOBAL` \| `SESSION`	yes
`character_set_database`	`GLOBAL` \| `SESSION`	yes
`character_set_results`	`GLOBAL` \| `SESSION`	yes
`character_sets_dir`	`GLOBAL`	no
`character_set_server`	`GLOBAL` \| `SESSION`	yes
`character_set_system`	`GLOBAL`	no
`collation_connection`	`GLOBAL` \| `SESSION`	yes
`collation_database`	`GLOBAL` \| `SESSION`	yes
`collation_server`	`GLOBAL` \| `SESSION`	yes
`concurrent_insert`	`GLOBAL`	yes
`connect_timeout`	`GLOBAL`	yes
`datadir`	`GLOBAL`	no
`date_format`	`GLOBAL` \| `SESSION`	yes
`datetime_format`	`GLOBAL` \| `SESSION`	yes
`debug`	`GLOBAL` \| `SESSION`	yes
`default-collation`		no
`default_week_format`	`GLOBAL` \| `SESSION`	yes
`delayed_insert_limit`	`GLOBAL`	yes
`delayed_insert_timeout`	`GLOBAL`	yes
`delayed_queue_size`	`GLOBAL`	yes
`delay_key_write`	`GLOBAL`	yes
`error_count`	`SESSION`	no
`expire_logs_days`	`GLOBAL`	yes
`flush`	`GLOBAL`	yes
`flush_time`	`GLOBAL`	yes
`foreign_key_checks`	`SESSION`	yes
`ft_boolean_syntax`	`GLOBAL`	yes
`ft_max_word_len`	`GLOBAL`	no
`ft_min_word_len`	`GLOBAL`	no
`ft_query_expansion_limit`	`GLOBAL`	no
`ft_stopword_file`	`GLOBAL`	no
`group_concat_max_len`	`GLOBAL` \| `SESSION`	yes
`have_archive`	`GLOBAL`	no
`have_bdb`	`GLOBAL`	no
`have_blackhole_engine`	`GLOBAL`	no
`have_compress`	`GLOBAL`	no
`have_crypt`	`GLOBAL`	no
`have_csv`	`GLOBAL`	no
`have_example_engine`	`GLOBAL`	no
`have_geometry`	`GLOBAL`	no
`have_innodb`	`GLOBAL`	no
`have_isam`	`GLOBAL`	no
`have_merge_engine`	`GLOBAL`	no
`have_ndbcluster`	`GLOBAL`	no
`have_openssl`	`GLOBAL`	no
`have_query_cache`	`GLOBAL`	no
`have_raid`	`GLOBAL`	no
`have_rtree_keys`	`GLOBAL`	no
`have_symlink`	`GLOBAL`	no
`identity`	`SESSION`	yes
`init_connect`	`GLOBAL`	yes
`init_file`	`GLOBAL`	no
`init_slave`	`GLOBAL`	yes
`innodb_additional_mem_pool_size`	`GLOBAL`	no
`innodb_autoextend_increment`	`GLOBAL`	yes
`innodb_buffer_pool_awe_mem_mb`	`GLOBAL`	no
`innodb_buffer_pool_size`	`GLOBAL`	no
`innodb_data_file_path`	`GLOBAL`	no
`innodb_data_home_dir`	`GLOBAL`	no
`innodb_fast_shutdown`	`GLOBAL`	yes
`innodb_file_io_threads`	`GLOBAL`	no
`innodb_file_per_table`	`GLOBAL`	no
`innodb_flush_log_at_trx_commit`	`GLOBAL`	yes
`innodb_flush_method`	`GLOBAL`	no
`innodb_force_recovery`	`GLOBAL`	no
`innodb_locks_unsafe_for_binlog`	`GLOBAL`	no
`innodb_lock_wait_timeout`	`GLOBAL`	no
`innodb_log_arch_dir`	`GLOBAL`	no
`innodb_log_archive`	`GLOBAL`	no
`innodb_log_buffer_size`	`GLOBAL`	no
`innodb_log_files_in_group`	`GLOBAL`	no
`innodb_log_file_size`	`GLOBAL`	no
`innodb_log_group_home_dir`	`GLOBAL`	no
`innodb_max_dirty_pages_pct`	`GLOBAL`	yes
`innodb_max_purge_lag`	`GLOBAL`	yes
`innodb_mirrored_log_groups`	`GLOBAL`	no
`innodb_open_files`	`GLOBAL`	no
`innodb_status_file`	`GLOBAL`	no
`innodb_table_locks`	`GLOBAL` \| `SESSION`	yes
`innodb_thread_concurrency`	`GLOBAL`	yes
`insert_id`	`SESSION`	yes
`interactive_timeout`	`GLOBAL` \| `SESSION`	yes
`isam`		no
`join_buffer_size`	`GLOBAL` \| `SESSION`	yes
`key_buffer_size`	`GLOBAL`	yes
`key_cache_age_threshold`	`GLOBAL`	yes
`key_cache_block_size`	`GLOBAL`	yes
`key_cache_division_limit`	`GLOBAL`	yes
`language`	`GLOBAL`	no
`last_insert_id`	`SESSION`	yes
`lc_time_names`	`GLOBAL` \| `SESSION`	yes
`license`	`GLOBAL`	no
`local_infile`	`GLOBAL`	yes
`locked_in_memory`	`GLOBAL`	no
`log`	`GLOBAL`	no
`log_bin`	`GLOBAL`	no
`log_error`	`GLOBAL`	no
`log_queries_not_using_indexes`	`GLOBAL`	yes
`log_slave_updates`	`GLOBAL`	no
`log_slow_queries`	`GLOBAL`	no
`log-warnings`	`GLOBAL` \| `SESSION`	yes
`long_query_time`	`GLOBAL` \| `SESSION`	yes
`lower_case_file_system`	`GLOBAL`	no
`lower_case_table_names`	`GLOBAL`	no
`low_priority_updates`	`GLOBAL` \| `SESSION`	yes
`max_allowed_packet`	`GLOBAL` \| `SESSION`	yes
`max_binlog_cache_size`	`GLOBAL`	yes
`max_binlog_size`	`GLOBAL`	yes
`max_connect_errors`	`GLOBAL`	yes
`max_connections`	`GLOBAL`	yes
`max_delayed_threads`	`GLOBAL` \| `SESSION`	yes
`max_error_count`	`GLOBAL` \| `SESSION`	yes
`max_heap_table_size`	`GLOBAL` \| `SESSION`	yes
`max_insert_delayed_threads`	`GLOBAL` \| `SESSION`	yes
`max_join_size`	`GLOBAL` \| `SESSION`	yes
`max_length_for_sort_data`	`GLOBAL` \| `SESSION`	yes
`max_prepared_stmt_count`	`GLOBAL`	yes
`max_relay_log_size`	`GLOBAL`	yes
`max_seeks_for_key`	`GLOBAL` \| `SESSION`	yes
`max_sort_length`	`GLOBAL` \| `SESSION`	yes
`max_tmp_tables`	`GLOBAL` \| `SESSION`	yes
`max_user_connections`	`GLOBAL`	yes
`max_write_lock_count`	`GLOBAL`	yes
`locked_in_memory`	`GLOBAL`	no
`myisam_block_size`	`GLOBAL` \| `SESSION`	yes
`myisam_data_pointer_size`	`GLOBAL`	yes
`myisam_max_extra_sort_file_size`	`GLOBAL`	no
`myisam_max_sort_file_size`	`GLOBAL`	yes
`myisam_recover_options`	`GLOBAL`	no
`myisam_repair_threads`	`GLOBAL` \| `SESSION`	yes
`myisam_sort_buffer_size`	`GLOBAL` \| `SESSION`	yes
`myisam_stats_method`	`GLOBAL` \| `SESSION`	yes
`named_pipe`	`GLOBAL`	no
`ndb_autoincrement_prefetch_sz`	`GLOBAL` \| `SESSION`	yes
`ndb_cache_check_time`	`GLOBAL`	yes
`ndbcluster`	`GLOBAL` \| `SESSION`	yes
`ndb_force_send`	`GLOBAL` \| `SESSION`	yes
`ndb_use_exact_count`	`GLOBAL` \| `SESSION`	yes
`net_buffer_length`	`GLOBAL` \| `SESSION`	yes
`net_read_timeout`	`GLOBAL` \| `SESSION`	yes
`net_retry_count`	`GLOBAL` \| `SESSION`	yes
`net_write_timeout`	`GLOBAL` \| `SESSION`	yes
`new`	`GLOBAL` \| `SESSION`	yes
`old_passwords`	`GLOBAL` \| `SESSION`	yes
`open_files_limit`	`GLOBAL`	no
`pid_file`	`GLOBAL`	no
`port`	`GLOBAL`	no
`preload_buffer_size`	`GLOBAL` \| `SESSION`	yes
`prepared_stmt_count`	`GLOBAL` \| `SESSION`	no
`protocol_version`	`GLOBAL`	no
`query_alloc_block_size`	`GLOBAL` \| `SESSION`	yes
`query_cache_limit`	`GLOBAL`	yes
`query_cache_min_res_unit`	`GLOBAL`	yes
`query_cache_size`	`GLOBAL`	yes
`query_cache_type`	`GLOBAL` \| `SESSION`	yes
`query_cache_wlock_invalidate`	`GLOBAL` \| `SESSION`	yes
`query_prealloc_size`	`GLOBAL` \| `SESSION`	yes
`range_alloc_block_size`	`GLOBAL` \| `SESSION`	yes
`read_buffer_size`	`GLOBAL` \| `SESSION`	yes
`read_only`	`GLOBAL`	yes
`read_rnd_buffer_size`	`GLOBAL` \| `SESSION`	yes
`relay_log_purge`	`GLOBAL`	yes
`relay_log_space_limit`	`GLOBAL`	no
`report_host`	`GLOBAL`	no
`report_password`	`GLOBAL`	no
`report_port`	`GLOBAL`	no
`report_user`	`GLOBAL`	no
`rpl_recovery_rank`	`GLOBAL`	yes
`safe_show_database`	`GLOBAL`	yes
`secure_auth`	`GLOBAL`	yes
`server_id`	`GLOBAL`	yes
`shared_memory`	`GLOBAL`	no
`shared_memory_base_name`	`GLOBAL`	no
`skip_external_locking`	`GLOBAL`	no
`skip-isam`		no
`skip_networking`	`GLOBAL`	no
`skip_show_database`	`GLOBAL`	no
`skip-sync-bdb-logs`	`GLOBAL`	no
`slave_compressed_protocol`	`GLOBAL`	yes
`slave_load_tmpdir`	`GLOBAL`	no
`slave_net_timeout`	`GLOBAL`	yes
`slave_skip_errors`	`GLOBAL`	no
`slave_transaction_retries`	`GLOBAL`	yes
`slow_launch_time`	`GLOBAL`	yes
`socket`	`GLOBAL`	no
`sort_buffer_size`	`GLOBAL` \| `SESSION`	yes
`sql_auto_is_null`	`SESSION`	yes
`sql_big_selects`	`GLOBAL` \| `SESSION`	yes
`sql_big_tables`	`SESSION`	yes
`sql_buffer_result`	`SESSION`	yes
`sql_log_bin`	`SESSION`	yes
`sql_log_off`	`SESSION`	yes
`sql_log_update`	`SESSION`	yes
`sql_low_priority_updates`	`GLOBAL` \| `SESSION`	yes
`sql_max_join_size`	`GLOBAL` \| `SESSION`	yes
`sql_mode`	`GLOBAL` \| `SESSION`	yes
`sql_notes`	`SESSION`	yes
`sql_quote_show_create`	`SESSION`	yes
`sql_safe_updates`	`SESSION`	yes
`sql_select_limit`	`SESSION`	yes
`sql_slave_skip_counter`	`GLOBAL`	yes
`sql_warnings`	`SESSION`	yes
`ssl_ca`	`GLOBAL`	no
`ssl_capath`	`GLOBAL`	no
`ssl_cert`	`GLOBAL`	no
`ssl_cipher`	`GLOBAL`	no
`ssl_key`	`GLOBAL`	no
`storage_engine`	`GLOBAL` \| `SESSION`	yes
`sync-bdb-logs`	`GLOBAL`	no
`sync_binlog`	`GLOBAL`	yes
`sync_frm`	`GLOBAL`	yes
`system_time_zone`	`GLOBAL`	no
`table_cache`	`GLOBAL`	yes
`table_type`	`GLOBAL` \| `SESSION`	yes
`thread_cache_size`	`GLOBAL`	yes
`thread_concurrency`	`GLOBAL`	no
`thread_stack`	`GLOBAL`	no
`time_format`	`GLOBAL` \| `SESSION`	yes
`timestamp`	`SESSION`	yes
`time_zone`	`GLOBAL` \| `SESSION`	yes
`tmpdir`	`GLOBAL`	no
`tmp_table_size`	`GLOBAL` \| `SESSION`	yes
`transaction_alloc_block_size`	`GLOBAL` \| `SESSION`	yes
`transaction_prealloc_size`	`GLOBAL` \| `SESSION`	yes
`tx_isolation`	`GLOBAL` \| `SESSION`	yes
`unique_checks`	`SESSION`	yes
`version`	`GLOBAL`	no
`version_comment`	`GLOBAL`	no
`version_compile_machine`	`GLOBAL`	no
`version_compile_os`	`GLOBAL`	no
`wait_timeout`	`GLOBAL` \| `SESSION`	yes
`warning_count`	`SESSION`	no

For additional system variable information, see these sections:

Section 5.1.4, “Session System Variables”, describes system variables that exist only as session variables (that is, they do not have any global counterpart).
Section 5.1.5, “Using System Variables”, discusses the syntax for setting and displaying system variable values.
Section 5.1.5.2, “Dynamic System Variables”, lists the variables that can be set at runtime.
Information on tuning system variables can be found in Section 7.5.2, “Tuning Server Parameters”.
Section 13.2.5, “InnoDB Startup Options and System Variables”, lists InnoDB system variables.
Section 15.4.3, “MySQL Cluster System Variables”, lists system variables which are specific to MySQL Cluster.

Note

Some of the following variable descriptions refer to “enabling” or “disabling” a variable. These variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. However, to set such a variable on the command line or in an option file, you must set it to 1 or 0; setting it to ON or OFF will not work. For example, on the command line, --delay_key_write=1 works but --delay_key_write=ON does not.

Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.

ansi_mode
This is ON if mysqld was started with --ansi. See Section 1.8.3, “Running MySQL in ANSI Mode”. This variable was added in MySQL 3.23.6 and removed in 3.23.41. See the description for sql_mode.
back_log
The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets very many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time.
In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix listen() system call should have more details. Check your OS documentation for the maximum value for this variable. back_log cannot be set higher than your operating system limit.
basedir
The MySQL installation base directory. This variable can be set with the --basedir option. Relative pathnames for other variables usually are resolved relative to the base directory.
bdb_cache_size
The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you do not use BDB tables, you should start mysqld with --skip-bdb to not allocate memory for this cache. This variable was added in MySQL 3.23.14.
bdb_home
The base directory for BDB tables. This should be assigned the same value as the datadir variable. This variable was added in MySQL 3.23.14.
bdb_log_buffer_size
The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you do not use BDB tables, you should set this to 0 or start mysqld with --skip-bdb in order not to allocate memory for this cache. This variable was added in MySQL 3.23.31.
bdb_logdir
The directory where the BDB storage engine writes its log files. This variable can be set with the --bdb-logdir option. This variable was added in MySQL 3.23.14.
bdb_max_lock
The maximum number of locks that can be active for a BDB table (10,000 by default). You should increase this value if errors such as the following occur when you perform long transactions or when mysqld has to examine many rows to calculate a query:
```
bdb: Lock table is out of available locks
Got error 12 from ...
```
This variable was added in MySQL 3.23.29.
bdb_shared_data
This is ON if you are using --bdb-shared-data to start Berkeley DB in multi-process mode. (Do not use DB_PRIVATE when initializing Berkeley DB.) This variable was added in MySQL 3.23.29.
bdb_tmpdir
The BDB temporary file directory. This variable was added in MySQL 3.23.14.
bdb_version
See the description for version_bdb.
binlog_cache_size
The size of the cache to hold the SQL statements for the binary log during a transaction. A binary log cache is allocated for each client if the server supports any transactional storage engines and, starting from MySQL 4.1.2, if the server has the binary log enabled (--log-bin option). If you often use large, multiple-statement transactions, you can increase this cache size to get more performance. The Binlog_cache_use and Binlog_cache_disk_use status variables can be useful for tuning the size of this variable. This variable was added in MySQL 3.23.29. See Section 5.3.4, “The Binary Log”.
MySQL Enterprise For recommendations on the optimum setting for binlog_cache_size subscribe to the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.
bulk_insert_buffer_size
MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA INFILE when adding data to non-empty tables. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. The default value is 8MB. Before MySQL 4.0.3. this variable was named myisam_bulk_insert_tree_size.
character_set
The default character set. This variable was added in MySQL 3.23.3, then removed in MySQL 4.1.1 and replaced by the various character_set_xxx variables.
character_set_client
The character set for statements that arrive from the client. This variable was added in MySQL 4.1.1.
The session value of this variable is set using the character set requested by the client when the client connects to the server. (Many clients support a --default-character-set option to enable this character set to be specified explicitly. See also Section 9.1.4, “Connection Character Sets and Collations”.) The global value of the variable is used to set the session value in cases when the client-requested value is unknown or not available, or the server is configured to ignore client requests:
- The client is from a version of MySQL older than MySQL 4.1, and thus does not request a character set.
- The client requests a character set not known to the server. For example, a Japanese-enabled client requests sjis when connecting to a server not configured with sjis support.
- mysqld was started with the --skip-character-set-client-handshake option, which causes it to ignore client character set configuration. This reproduces MySQL 4.0 behavior and is useful should you wish to upgrade the server without upgrading all the clients.
character_set_connection
The character set used for literals that do not have a character set introducer and for number-to-string conversion. This variable was added in MySQL 4.1.1.
character_set_database
The character set used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as character_set_server. This variable was added in MySQL 4.1.1.
character_set_results
The character set used for returning query results to the client. This variable was added in MySQL 4.1.1.
character_set_server
The server default character set. This variable was added in MySQL 4.1.1.
character_set_system
The character set used by the server for storing identifiers. The value is always utf8. This variable was added in MySQL 4.1.1.
character_sets
The supported character sets. This variable was added in MySQL 3.23.15 and removed in MySQL 4.1.1. (Use SHOW CHARACTER SET for a list of character sets.)
character_sets_dir
The directory where character sets are installed. This variable was added in MySQL 4.1.2.
collation_connection
The collation of the connection character set. This variable was added in MySQL 4.1.1.
collation_database
The collation used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as collation_server. This variable was added in MySQL 4.1.1.
collation_server
The server default collation. This variable was added in MySQL 4.1.1.
concurrent_insert
If ON (the default), MySQL allows INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle of the data file. You can turn this option off by starting mysqld with --safe or --skip-new. This variable was added in MySQL 3.23.7.
See also Section 7.3.3, “Concurrent Inserts”.
connect_timeout
The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake. The default value is 5 seconds.
Increasing the connect_timeout value might help if clients frequently encounter errors of the form Lost connection to MySQL server at 'XXX', system error: errno.
convert_character_set
The current character set mapping that was set by SET CHARACTER SET. This variable was removed in MySQL 4.1.
datadir
The MySQL data directory. This variable can be set with the --datadir option.
date_format
This variable is unused.
datetime_format
This variable is unused.
default_week_format
The default mode value to use for the WEEK() function. See Section 11.6, “Date and Time Functions”. This variable is available as of MySQL 4.0.14.

delay_key_write

This option applies only to MyISAM tables. It can have one of the following values to affect handling of the DELAY_KEY_WRITE table option that can be used in CREATE TABLE statements.

Option	Description
`OFF`	`DELAY_KEY_WRITE` is ignored.
`ON`	MySQL honors any `DELAY_KEY_WRITE` option specified in `CREATE TABLE` statements. This is the default value.
`ALL`	All new opened tables are treated as if they were created with the `DELAY_KEY_WRITE` option enabled.

If DELAY_KEY_WRITE is enabled for a table, the key buffer is not flushed for the table on every index update, but only when the table is closed. This speeds up writes on keys a lot, but if you use this feature, you should add automatic checking of all MyISAM tables by starting the server with the --myisam-recover option (for example, --myisam-recover=BACKUP,FORCE). See Section 5.1.2, “Command Options”, and Section 13.1.1, “MyISAM Startup Options”.

Note that if you enable external locking with --external-locking, there is no protection against index corruption for tables that use delayed key writes.

This variable was added in MySQL 3.23.8.

delayed_insert_limit
After inserting delayed_insert_limit delayed rows, the INSERT DELAYED handler thread checks whether there are any SELECT statements pending. If so, it allows them to execute before continuing to insert delayed rows.
delayed_insert_timeout
How many seconds an INSERT DELAYED handler thread should wait for INSERT statements before terminating.
delayed_queue_size
This is a per-table limit on the number of rows to queue when handling INSERT DELAYED statements. If the queue becomes full, any client that issues an INSERT DELAYED statement waits until there is room in the queue again.
expire_logs_days
The number of days for automatic binary log removal. The default is 0, which means “no automatic removal.” Possible removals happen at startup and at binary log rotation. This variable was added in MySQL 4.1.0.
flush
If ON, the server flushes (synchronizes) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section A.1.4.2, “What to Do If MySQL Keeps Crashing”. This variable is set to ON if you start mysqld with the --flush option. This variable was added in MySQL 3.22.9.
flush_time
If this is set to a non-zero value, all tables are closed every flush_time seconds to free up resources and synchronize unflushed data to disk. We recommend that this option be used only on Windows 9x or Me, or on systems with minimal resources. This variable was added in MySQL 3.22.18.
ft_boolean_syntax
The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. See Section 11.8.2, “Boolean Full-Text Searches”. This variable was added as a read-only variable in MySQL 4.0.1. It can be modified as of MySQL 4.1.2.
The default variable value is '+ -><()~*:""&|'. The rules for changing the value are as follows:
- Operator function is determined by position within the string.
- The replacement value must be 14 characters.
- Each character must be an ASCII non-alphanumeric character.
- Either the first or second character must be a space.
- No duplicates are allowed except the phrase quoting operators in positions 11 and 12. These two characters are not required to be the same, but they are the only two that may be.
- Positions 10, 13, and 14 (which by default are set to “:”, “&”, and “|”) are reserved for future extensions.
ft_max_word_len
The maximum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0.
Note
FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
ft_min_word_len
The minimum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0.
Note
FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
ft_query_expansion_limit
The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION. This variable was added in MySQL 4.1.1.
ft_stopword_file
The file from which to read the list of stopwords for full-text searches. All the words from the file are used; comments are not honored. By default, a built-in list of stopwords is used (as defined in the myisam/ft_static.c file). Setting this variable to the empty string ('') disables stopword filtering. This variable was added in MySQL 4.0.10.
Note
FULLTEXT indexes must be rebuilt after changing this variable or the contents of the stopword file. Use REPAIR TABLE tbl_name QUICK.
group_concat_max_len
The maximum allowed result length in bytes for the GROUP_CONCAT() function. The default is 1024. This variable was added in MySQL 4.1.0.
have_archive
YES if mysqld supports ARCHIVE tables, NO if not. This variable was added in MySQL 4.1.3.
have_bdb
YES if mysqld supports BDB tables. DISABLED if --skip-bdb is used. This variable was added in MySQL 3.23.30.
have_blackhole_engine
YES if mysqld supports BLACKHOLE tables, NO if not. This variable was added in MySQL 4.1.11.
have_compress
YES if the zlib compression library is available to the server, NO if not. If not, the COMPRESS() and UNCOMPRESS() functions cannot be used. This variable was added in MySQL 4.1.1.
have_crypt
YES if the crypt() system call is available to the server, NO if not. If not, the ENCRYPT() function cannot be used. This variable was added in MySQL 4.0.10.
have_csv
YES if mysqld supports ARCHIVE tables, NO if not. This variable was added in MySQL 4.1.4.
have_example_engine
YES if mysqld supports EXAMPLE tables, NO if not. This variable was added in MySQL 4.1.4.
have_geometry
YES if the server supports spatial data types, NO if not. This variable was added in MySQL 4.1.3.
have_innodb
YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used. This variable was added in MySQL 3.23.37.
have_isam
YES if mysqld supports ISAM tables. DISABLED if --skip-isam is used. This variable was added in MySQL 3.23.30.
have_merge_engine
YES if mysqld supports MERGE tables. DISABLED if --skip-merge is used. This variable was added in MySQL 4.1.21.
have_openssl
YES if mysqld supports SSL (encryption) connections, NO if not. This variable was added in MySQL 3.23.43.
have_query_cache
YES if mysqld supports the query cache, NO if not. This variable was added in MySQL 4.0.2.
have_raid
YES if mysqld supports the RAID option, NO if not. This variable was added in MySQL 3.23.30.
have_rtree_keys
YES if RTREE indexes are available, NO if not. (These are used for spatial indexes in MyISAM tables.) This variable was added in MySQL 4.1.3.
have_symlink
YES if symbolic link support is enabled, NO if not. This is required on Unix for support of the DATA DIRECTORY and INDEX DIRECTORY table options, and on Windows for support of data directory symlinks.
This variable was added in MySQL 4.0.0.
init_connect
A string to be executed by the server for each client that connects. The string consists of one or more SQL statements. To specify multiple statements, separate them by semicolon characters. For example, each client begins by default with autocommit mode enabled. There is no global system variable to specify that autocommit should be disabled by default, but init_connect can be used to achieve the same effect:
```
SET GLOBAL init_connect='SET AUTOCOMMIT=0';
```
This variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:
```
[mysqld]
init_connect='SET AUTOCOMMIT=0'
```
Note that the content of init_connect is not executed for users that have the SUPER privilege. This is done so that an erroneous value for init_connect does not prevent all clients from connecting. For example, the value might contain a statement that has a syntax error, thus causing client connections to fail. Not executing init_connect for users that have the SUPER privilege enables them to open a connection and fix the init_connect value.
This variable was added in MySQL 4.1.2.
init_file
The name of the file specified with the --init-file option when you start the server. This should be a file containing SQL statements that you want the server to execute when it starts. Each statement must be on a single line and should not include comments. This variable was added in MySQL 3.23.2.
init_slave
This variable is similar to init_connect, but is a string to be executed by a slave server each time the SQL thread starts. The format of the string is the same as for the init_connect variable. This variable was added in MySQL 4.1.2.
innodb_xxx
InnoDB system variables are listed in Section 13.2.5, “InnoDB Startup Options and System Variables”.
interactive_timeout
The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also wait_timeout.
join_buffer_size
The size of the buffer that is used for joins that do not use indexes and thus perform full table scans. Normally, the best way to get fast joins is to add indexes. Increase the value of join_buffer_size to get a faster full join when adding indexes is not possible. One join buffer is allocated for each full join between two tables. For a complex join between several tables for which indexes are not used, multiple join buffers might be necessary.
key_buffer_size
Index blocks for MyISAM and ISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache.
The maximum allowable setting for key_buffer_size is 4GB. The effective maximum size might be less, depending on your available physical RAM and per-process RAM limits imposed by your operating system or hardware platform. The value of this variable indicates the amount of memory requested. Internally, the server allocates as much memory as possible up to this amount, but the actual allocation might be less.
Increase the value to get better index handling (for all reads and multiple writes) to as much as you can afford. Using a value that is 25% of total memory on a machine that mainly runs MySQL is quite common. However, if you make the value too large (for example, more than 50% of your total memory) your system might start to page and become extremely slow. MySQL relies on the operating system to perform filesystem caching for data reads, so you must leave some room for the filesystem cache. Consider also the memory requirements of other storage engines.
For even more speed when writing many rows at the same time, use LOCK TABLES. See Section 7.2.13, “Speed of INSERT Statements”.
You can check the performance of the key buffer by issuing a SHOW STATUS statement and examining the Key_read_requests, Key_reads, Key_write_requests, and Key_writes status variables. (See Section 12.5.4, “SHOW Syntax”.) The Key_reads/Key_read_requests ratio should normally be less than 0.01. The Key_writes/Key_write_requests ratio is usually near 1 if you are using mostly updates and deletes, but might be much smaller if you tend to do updates that affect many rows at the same time or if you are using the DELAY_KEY_WRITE table option.
The fraction of the key buffer in use can be determined using key_buffer_size in conjunction with the Key_blocks_unused status variable and the buffer block size. From MySQL 4.1.1 on, the buffer block size is available from the key_cache_block_size server variable. The fraction of the buffer in use is:
```
1 - ((Key_blocks_unused × key_cache_block_size) / key_buffer_size)
```
This value is an approximation because some space in the key buffer may be allocated internally for administrative structures.
Before MySQL 4.1.1, key cache blocks are 1024 bytes, and before MySQL 4.1.2, Key_blocks_unused is unavailable. The Key_blocks_used variable can be used as follows to determine the fraction of the key buffer in use:
```
(Key_blocks_used × 1024) / key_buffer_size
```
However, Key_blocks_used indicates the maximum number of blocks that have ever been in use at once, so this formula does not necessarily represent the current fraction of the buffer that is in use.
As of MySQL 4.1, it is possible to create multiple MyISAM key caches. The size limit of 4GB applies to each cache individually, not as a group. See Section 7.4.6, “The MyISAM Key Cache”.
key_cache_age_threshold
This value controls the demotion of buffers from the hot sub-chain of a key cache to the warm sub-chain. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. This variable was added in MySQL 4.1.1. See Section 7.4.6, “The MyISAM Key Cache”.
key_cache_block_size
The size in bytes of blocks in the key cache. The default value is 1024. This variable was added in MySQL 4.1.1. See Section 7.4.6, “The MyISAM Key Cache”.
key_cache_division_limit
The division point between the hot and warm sub-chains of the key cache buffer chain. The value is the percentage of the buffer chain to use for the warm sub-chain. Allowable values range from 1 to 100. The default value is 100. This variable was added in MySQL 4.1.1. See Section 7.4.6, “The MyISAM Key Cache”.
language
The language used for error messages.
large_files_support
Whether mysqld was compiled with options for large file support. This variable was added in MySQL 3.23.28.
lc_time_names
This variable specifies the locale that controls the language used to display day and month names and abbreviations. This variable affects the output from the DATE_FORMAT(), DAYNAME() and MONTHNAME() functions. Locale names are POSIX-style values such as 'ja_JP' or 'pt_BR'. The default value is 'en_US' regardless of your system's locale setting. For further information, see Section 9.8, “MySQL Server Locale Support”. This variable was added in MySQL 4.1.21.
license
The type of license the server has. This variable was added in MySQL 4.0.19.
local_infile
Whether LOCAL is supported for LOAD DATA INFILE statements. See Section 5.4.4, “Security Issues with LOAD DATA LOCAL”. This variable was added in MySQL 4.0.3.
locked_in_memory
Whether mysqld was locked in memory with --memlock. This variable was added in MySQL 3.23.25.
log
Whether logging of all statements to the general query log is enabled. See Section 5.3.2, “The General Query Log”.
log_bin
Whether the binary log is enabled. This variable was added in MySQL 3.23.14. See Section 5.3.4, “The Binary Log”.
log_error
The location of the error log. This variable was added in MySQL 4.0.10.
log_slave_updates
Whether updates received by a slave server from a master server should be logged to the slave's own binary log. Binary logging must be enabled on the slave for this variable to have any effect. This variable was added in MySQL 3.23.17. See Section 14.8, “Replication Startup Options”.
log_slow_queries
Whether slow queries should be logged. “Slow” is determined by the value of the long_query_time variable. This variable was added in MySQL 4.0.2. See Section 5.3.5, “The Slow Query Log”.
log_update
Whether the update log is enabled. This variable was added in MySQL 3.22.18. Note that the binary log is preferable to the update log, which is unavailable as of MySQL 5.0. See Section 5.3.3, “The Update Log”.
log_warnings
Whether to produce additional warning messages. This variable was added in MySQL 4.0.3. It is enabled by default as of MySQL 4.0.19 and 4.1.2. As of MySQL 4.0.21 and 4.1.3, the variable can take values greater than 1 and aborted connections are not logged to the error log unless the value is greater than 1.
long_query_time
If a query takes longer than this many seconds, the server increments the Slow_queries status variable. If you are using the --log-slow-queries option, the query is logged to the slow query log file. This value is measured in real time, not CPU time, so a query that is under the threshold on a lightly loaded system might be above the threshold on a heavily loaded one. The minimum value is 1. The default is 10. See Section 5.3.5, “The Slow Query Log”.
low_priority_updates
If set to 1, all INSERT, UPDATE, DELETE, and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table. This affects only storage engines that use only table-level locking (MyISAM, MEMORY, MERGE). Before MySQL 3.22.5, this variable was named sql_low_priority_updates.
lower_case_file_system
This variable describes the case sensitivity of filenames on the filesystem where the data directory is located. OFF means filenames are case sensitive, ON means they are not case sensitive. This variable was added in MySQL 4.0.19.
lower_case_table_names
If set to 1 table names are stored in lowercase on disk and table name comparisons are not case sensitive. This variable was added in MySQL 3.23.6. If set to 2 (new in 4.0.18), table names are stored as given but compared in lowercase. From MySQL 4.0.2, this option also applies to database names. From 4.1.1, it also applies to table aliases. See Section 8.2.2, “Identifier Case Sensitivity”.
Note
If you are using InnoDB tables, you should set this variable to 1 on all platforms to force names to be converted to lowercase.
You should not set this variable to 0 if you are running MySQL on a system that does not have case-sensitive filenames (such as Windows or Mac OS X). New in 4.0.18: If this variable is not set at startup and the filesystem on which the data directory is located does not have case-sensitive filenames, MySQL automatically sets lower_case_table_names to 2.
max_allowed_packet
The maximum size of one packet or any generated/intermediate string.
The packet message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small, to catch large (possibly incorrect) packets.
You must increase this value if you are using large BLOB columns or long strings. It should be as big as the largest BLOB you want to use. The protocol limit for max_allowed_packet is 16MB before MySQL 4.0 and 1GB thereafter. The value should be a multiple of 1024; non-multiples are rounded down to the nearest multiple.
max_binlog_cache_size
If a multiple-statement transaction requires more than this many bytes of memory, the server generates a Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage error. The minimum value is 4096, the maximum and default values are 4GB. This variable was added in MySQL 3.23.29.
max_binlog_size
If a write to the binary log causes the current log file size to exceed the value of this variable, the server rotates the binary logs (closes the current file and opens the next one). You cannot set this variable to more than 1GB or to less than 4096 bytes. (The minimum before MYSQL 4.0.14 is 1024 bytes.) The default value is 1GB. This variable was added in MySQL 3.23.33.
A transaction is written in one chunk to the binary log, so it is never split between several binary logs. Therefore, if you have big transactions, you might see binary logs larger than max_binlog_size.
If max_relay_log_size is 0, the value of max_binlog_size applies to relay logs as well. max_relay_log_size was added in MySQL 4.0.14.
max_connect_errors
If there are more than this number of interrupted connections from a host, that host is blocked from further connections. You can unblock blocked hosts with the FLUSH HOSTS statement.
max_connections
The number of simultaneous client connections allowed. By default, this is 100. See Section A.1.2.7, “Too many connections”, for more information.
MySQL Enterprise For notification that the maximum number of connections is getting dangerously high and for advice on setting the optimum value for max_connections subscribe to the MySQL Enterprise Monitor. For more information see http://www.mysql.com/products/enterprise/advisors.html.
Increasing this value increases the number of file descriptors that mysqld requires. See Section 7.4.8, “How MySQL Opens and Closes Tables”, for comments on file descriptor limits.
max_delayed_threads
Do not start more than this number of threads to handle INSERT DELAYED statements. If you try to insert data into a new table after all INSERT DELAYED threads are in use, the row is inserted as if the DELAYED attribute wasn't specified. If you set this to 0, MySQL never creates a thread to handle DELAYED rows; in effect, doing so disables DELAYED entirely. This variable was added in MySQL 3.23.0.
For the SESSION value of this variable, the only valid values are 0 or the GLOBAL value.
max_error_count
The maximum number of error, warning, and note messages to be stored for display by the SHOW ERRORS or SHOW WARNINGS statements. This variable was added in MySQL 4.1.0.
max_heap_table_size
This variable sets the maximum size to which MEMORY (HEAP) tables are allowed to grow. The value of the variable is used to calculate MEMORY table MAX_ROWS values. Setting this variable has no effect on any existing MEMORY table, unless the table is re-created with a statement such as CREATE TABLE, or altered with ALTER TABLE or TRUNCATE TABLE. A server restart also sets the maximum size of existing MEMORY tables to the global max_heap_table_size value. This variable was added in MySQL 3.23.0.
MySQL Enterprise Subscribers to the MySQL Enterprise Monitor receive recommendations for the optimum setting for max_heap_table_size. For more information see http://www.mysql.com/products/enterprise/advisors.html.
max_insert_delayed_threads
This variable is a synonym for max_delayed_threads. It was added in MySQL 4.0.19.
max_join_size
Do not allow SELECT statements that probably need to examine more than max_join_size rows (for single-table statements) or row combinations (for multiple-table statements) or that are likely to do more than max_join_size disk seeks. By setting this value, you can catch SELECT statements where keys are not used properly and that would probably take a long time. Set it if your users tend to perform joins that lack a WHERE clause, that take a long time, or that return millions of rows.
Setting this variable to a value other than DEFAULT resets the value of SQL_BIG_SELECTS to 0. If you set the SQL_BIG_SELECTS value again, the max_join_size variable is ignored.
If a query result is in the query cache, no result size check is performed, because the result has previously been computed and it does not burden the server to send it to the client.
This variable previously was named sql_max_join_size.
max_length_for_sort_data
The cutoff on the size of index values that determines which filesort algorithm to use. See Section 7.2.8, “ORDER BY Optimization”. This variable was added in MySQL 4.1.1
max_prepared_stmt_count
This variable limits the total number of prepared statements in the server. It can be used in environments where there is the potential for denial-of-service attacks based on running the server out of memory by preparing huge numbers of statements. The default value is 16,382. The allowable range of values is from 0 to 1 million. If the value is set lower than the current number of prepared statements, existing statements are not affected and can be used, but no new statements can be prepared until the current number drops below the limit. This variable was added in MySQL 4.1.19.
max_relay_log_size
If a write by a replication slave to its relay log causes the current log file size to exceed the value of this variable, the slave rotates the relay logs (closes the current file and opens the next one). If max_relay_log_size is 0, the server uses max_binlog_size for both the binary log and the relay log. If max_relay_log_size is greater than 0, it constrains the size of the relay log, which enables you to have different sizes for the two logs. You must set max_relay_log_size to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. This variable was added in MySQL 4.0.14. See Section 14.3, “Replication Implementation Details”.
max_seeks_for_key
Limit the assumed maximum number of seeks when looking up rows based on a key. The MySQL optimizer assumes that no more than this number of key seeks are required when searching for matching rows in a table by scanning an index, regardless of the actual cardinality of the index (see Section 12.5.4.11, “SHOW INDEX Syntax”). By setting this to a low value (say, 100), you can force MySQL to prefer indexes instead of table scans.
This variable was added in MySQL 4.0.14.
max_sort_length
The number of bytes to use when sorting BLOB or TEXT values. Only the first max_sort_length bytes of each value are used; the rest are ignored.
max_tmp_tables
The maximum number of temporary tables a client can keep open at the same time. (This option does not yet do anything.)
max_user_connections
The maximum number of simultaneous connections allowed to any given MySQL account. A value of 0 means “no limit.” This variable was added in MySQL 3.23.34.
This variable has only a global form.
max_write_lock_count
After this many write locks, allow some pending read lock requests to be processed in between. This variable was added in MySQL 3.23.7.
myisam_block_size
The block size to be used for MyISAM index pages.
myisam_data_pointer_size
The default pointer size in bytes, to be used by CREATE TABLE for MyISAM tables when no MAX_ROWS option is specified. This variable cannot be less than 2 or larger than 7. The default value is 4. This variable was added in MySQL 4.1.2. See Section A.1.2.12, “The table is full”.
myisam_max_extra_sort_file_size
If the temporary file used for fast MyISAM index creation would be larger than using the key cache by the amount specified here, prefer the key cache method. This is mainly used to force long character keys in large tables to use the slower key cache method to create the index. This variable was added in MySQL 3.23.37.
Note
The value is given in megabytes before 4.0.3 and in bytes thereafter.
myisam_max_sort_file_size
The maximum size of the temporary file that MySQL is allowed to use while re-creating a MyISAM index (during REPAIR TABLE, ALTER TABLE, or LOAD DATA INFILE). If the file size would be larger than this value, the index is created using the key cache instead, which is slower. This variable was added in MySQL 3.23.37.
Note
The value is given in megabytes before 4.0.3 and in bytes thereafter.
The default value is 2GB. If MyISAM index files exceed this size and disk space is available, increasing the value may help performance.
myisam_recover_options
The value of the --myisam-recover option. See Section 5.1.2, “Command Options”. This variable was added in MySQL 3.23.36.
myisam_repair_threads
If this value is greater than 1, MyISAM table indexes are created in parallel (each index in its own thread) during the Repair by sorting process. The default value is 1.
Note
Multi-threaded repair is still beta-quality code. This variable was added in MySQL 4.0.13.
myisam_sort_buffer_size
The size of the buffer that is allocated when sorting MyISAM indexes during a REPAIR TABLE or when creating indexes with CREATE INDEX or ALTER TABLE. This variable was added in MySQL 3.23.16.
myisam_stats_method
How the server treats NULL values when collecting statistics about the distribution of index values for MyISAM tables. This variable has two possible values, nulls_equal and nulls_unequal. For nulls_equal, all NULL index values are considered equal and form a single value group that has a size equal to the number of NULL values. For nulls_unequal, NULL values are considered unequal, and each NULL forms a distinct value group of size 1.
The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 7.4.7, “MyISAM Index Statistics Collection”.
Any unique prefix of a valid value may be used to set the value of this variable.
This variable was added in MySQL 4.1.15. For older versions, the statistics collection method is equivalent to nulls_equal.
named_pipe
On Windows, indicates whether the server supports connections over named pipes. This variable was added in MySQL 3.23.50.
net_buffer_length
Each client thread is associated with a connection buffer and result buffer. Both begin with a size given by net_buffer_length but are dynamically enlarged up to max_allowed_packet bytes as needed. The result buffer shrinks to net_buffer_length after each SQL statement.
This variable should not normally be changed, but if you have very little memory, you can set it to the expected length of statements sent by clients. If statements exceed this length, the connection buffer is automatically enlarged. The maximum value to which net_buffer_length can be set is 1MB.
net_read_timeout
The number of seconds to wait for more data from a connection before aborting the read. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory. When the server is reading from the client, net_read_timeout is the timeout value controlling when to abort. When the server is writing to the client, net_write_timeout is the timeout value controlling when to abort. See also slave_net_timeout. This variable was added in MySQL 3.23.20.
net_retry_count
If a read on a communication port is interrupted, retry this many times before giving up. This value should be set quite high on FreeBSD because internal interrupts are sent to all threads. This variable was added in MySQL 3.23.7.
net_write_timeout
The number of seconds to wait for a block to be written to a connection before aborting the write. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory. See also net_read_timeout. This variable was added in MySQL 3.23.20.
new
This variable is used in MySQL 4.0 to turn on some 4.1 behaviors. This variable was added in MySQL 4.0.12.
old_passwords
Whether the server should use pre-4.1-style passwords for MySQL user accounts. This variable was added in MySQL 4.1.1.
one_shot
This is not a variable, but it can be used when setting some variables. It is described in Section 12.5.3, “SET Syntax”.
open_files_limit
The number of files that the operating system allows mysqld to open. This is the real value allowed by the system and might be different from the value you gave using the --open-files-limit option to mysqld or mysqld_safe. The value is 0 on systems where MySQL can't change the number of open files. This variable was added in MySQL 3.23.20.
pid_file
The pathname of the process ID (PID) file. This variable can be set with the --pid-file option. This variable was added in MySQL 3.23.23.
port
The number of the port on which the server listens for TCP/IP connections. This variable can be set with the --port option.
preload_buffer_size
The size of the buffer that is allocated when preloading indexes. This variable was added in MySQL 4.1.1.
prepared_stmt_count
The current number of prepared statements. (The maximum number of statements is given by the max_prepared_stmt_count system variable.) This variable was added in MySQL 4.1.19. In MySQL 4.1.23, it was converted to the global Prepared_stmt_count status variable.
protocol_version
The version of the client/server protocol used by the MySQL server. This variable was added in MySQL 3.23.18.
query_alloc_block_size
The allocation size of memory blocks that are allocated for objects created during statement parsing and execution. If you have problems with memory fragmentation, it might help to increase this a bit. This variable was added in MySQL 4.0.16.
query_cache_limit
Don't cache results that are larger than this number of bytes. The default value is 1MB. This variable was added in MySQL 4.0.1.
query_cache_min_res_unit
The minimum size for blocks allocated by the query cache. The default value is 4KB. Tuning information for this variable is given in Section 7.5.3.3, “Query Cache Configuration”. This variable is present from MySQL 4.1.
query_cache_size
The amount of memory allocated for caching query results. The default value is 0, which disables the query cache. The allowable values are multiples of 1024; other values are rounded down to the nearest multiple. Note that query_cache_size bytes of memory are allocated even if query_cache_type is set to 0. This variable was added in MySQL 4.0.1.
The query cache needs a minimum size of about 40KB to allocate its structures. (The exact size depends on system architecture.) If you set the value of query_cache_size too small, you'll get a warning, as described in Section 7.5.3.3, “Query Cache Configuration”.

query_cache_type

Set the query cache type. Setting the GLOBAL value sets the type for all clients that connect thereafter. Individual clients can set the SESSION value to affect their own use of the query cache.

Option	Description
`0` or `OFF`	Don't cache results in or retrieve results from the query cache. Note that this does not deallocate the query cache buffer. To do that, you should set `query_cache_size` to 0.
`1` or `ON`	Cache all cacheable query results except for those that begin with `SELECT SQL_NO_CACHE`.
`2` or `DEMAND`	Cache results only for cacheable queries that begin with `SELECT SQL_CACHE`.

This variable defaults to ON.

Any unique prefix of a valid value may be used to set the value of this variable.

This variable was added in MySQL 4.0.3.

query_cache_wlock_invalidate
Normally, when one client acquires a WRITE lock on a MyISAM table, other clients are not blocked from issuing statements that read from the table if the query results are present in the query cache. Setting this variable to 1 causes acquisition of a WRITE lock for a table to invalidate any queries in the query cache that refer to the table. This forces other clients that attempt to access the table to wait while the lock is in effect. This variable was added in MySQL 4.0.19.
query_prealloc_size
The size of the persistent buffer used for statement parsing and execution. This buffer is not freed between statements. If you are running complex queries, a larger query_prealloc_size value might be helpful in improving performance, because it can reduce the need for the server to perform memory allocation during query execution operations.
This variable was added in MySQL 4.0.16.
range_alloc_block_size
The size of blocks that are allocated when doing range optimization. This variable was added in MySQL 4.0.16.
read_buffer_size
Each thread that does a sequential scan allocates a buffer of this size (in bytes) for each table it scans. If you do many sequential scans, you might want to increase this value.
read_buffer_size and read_rnd_buffer_size are not specific to any storage engine and apply in a general manner for optimization. See Section 7.5.7, “How MySQL Uses Memory”, for example.
Before MySQL 4.0.3, this variable was named record_buffer.
read_only
This variable is off by default. When it is enabled, the server allows no updates except from users that have the SUPER privilege or (on a slave server) from updates performed by slave threads. On a slave server, this can be useful to ensure that the slave accepts updates only from its master server and not from clients.
read_only exists only as a GLOBAL variable, so changes to its value require the SUPER privilege. Changes to read_only on a master server are not replicated to slave servers. The value can be set on a slave server independent of the setting on the master.
This variable was added in MySQL 4.0.14.
read_rnd_buffer_size
When reading rows in sorted order following a key-sorting operation, the rows are read through this buffer to avoid disk seeks. See Section 7.2.8, “ORDER BY Optimization”. Setting the variable to a large value can improve ORDER BY performance by a lot. However, this is a buffer allocated for each client, so you should not set the global variable to a large value. Instead, change the session variable only from within those clients that need to run large queries.
read_buffer_size and read_rnd_buffer_size are not specific to any storage engine and apply in a general manner for optimization. See Section 7.5.7, “How MySQL Uses Memory”, for example.
Before MySQL 4.0.3, this variable was named record_rnd_buffer.
relay_log_purge
Disables or enables automatic purging of relay logs as soon as they are not needed any more. The default value is 1 (ON). This variable was added in MySQL 4.1.1.
relay_log_space_limit
Option Sets Variable Yes, relay_log_space_limit
Variable Name relay_log_space_limit
Variable Scope Global
Dynamic Variable No
Value Set
Type numeric
Default 0
The maximum amount of space to use for all relay logs.
rpl_recovery_rank
This variable is unused.
safe_show_database
Do not show databases for which the user has no database or table privileges. This can improve security if you are concerned about people being able to see what databases other users have. See also skip_show_database.
This variable was removed in MySQL 4.0.5. Beginning with this version, you should instead use the SHOW DATABASES privilege to control access by MySQL accounts to databases.
secure_auth
If the MySQL server has been started with the --secure-auth option, it blocks connections from all accounts that have passwords stored in the old (pre-4.1) format. In that case, the value of this variable is ON, otherwise it is OFF.
You should enable this option if you want to prevent all use of passwords in the old format (and hence insecure communication over the network). This variable was added in MySQL 4.1.1.
Server startup fails with an error if this option is enabled and the privilege tables are in pre-4.1 format.
server_id
The server ID. This value is set by the --server-id option. It is used for replication to enable master and slave servers to identify themselves uniquely. This variable was added in MySQL 3.23.26.
shared_memory
(Windows only.) Whether the server allows shared-memory connections. This variable was added in MySQL 4.1.1.
shared_memory_base_name
(Windows only.) The name of shared memory to use for shared-memory connections. This is useful when running multiple MySQL instances on a single physical machine. This variable was added in MySQL 4.1.0.
skip_external_locking
This is OFF if mysqld uses external locking, ON if external locking is disabled. Before MySQL 4.0.3, this variable was named skip_locking.
skip_networking
This is ON if the server allows only local (non-TCP/IP) connections. On Unix, local connections use a Unix socket file. On Windows, local connections use a named pipe or shared memory. On NetWare, only TCP/IP connections are supported, so do not set this variable to ON. This variable can be set to ON with the --skip-networking option. This variable was added in MySQL 3.22.23.
skip_show_database
This prevents people from using the SHOW DATABASES statement if they do not have the SHOW DATABASES privilege. This can improve security if you are concerned about people being able to see what databases other users have. See also safe_show_database. This variable was added in MySQL 3.23.4. As of MySQL 4.0.2, its effect also depends on the SHOW DATABASES privilege: If the variable value is ON, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. If the value is OFF, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database. Note that any global privilege is a privilege for the database.
slave_compressed_protocol
Whether to use compression of the master/slave protocol if both the slave and the master support it. This variable was added in MySQL 4.0.3.
slave_load_tmpdir
The name of the directory where the slave creates temporary files for replicating LOAD DATA INFILE statements. This variable was added in MySQL 4.0.0.
slave_net_timeout
The number of seconds to wait for more data from a master/slave connection before aborting the read. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory. This variable was added in MySQL 3.23.40.
slave_skip_errors
Normally, replication stops when an error occurs on the slave. This gives you the opportunity to resolve the inconsistency in the data manually. This variable tells the slave SQL thread to continue replication when a statement returns any of the errors listed in the variable value. This variable was added in MySQL 3.23.47.
slave_transaction_retries
If a replication slave SQL thread fails to execute a transaction because of an InnoDB deadlock or InnoDB's innodb_lock_wait_timeout or NDB Cluster's TransactionDeadlockDetectionTimeout or TransactionInactiveTimeout was exceeded, it automatically retries slave_transaction_retries times before stopping with an error. The default in MySQL 4.1 is 0. You must explicitly set the value to greater than 0 to enable the “retry” behavior, which is probably a good idea.
slow_launch_time
If creating a thread takes longer than this many seconds, the server increments the Slow_launch_threads status variable. This variable was added in MySQL 3.23.15.
socket
On Unix platforms, this variable is the name of the socket file that is used for local client connections. The default is /tmp/mysql.sock. (For some distribution formats, the directory might be different, such as /var/lib/mysql for RPMs.)
On Windows, this variable is the name of the named pipe that is used for local client connections. The default value is MySQL (not case sensitive).
sort_buffer_size
Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster ORDER BY or GROUP BY operations. See Section A.1.4.4, “Where MySQL Stores Temporary Files”.
sql_mode
The current server SQL mode. This variable was added in MySQL 3.23.41. It can be set dynamically as of MySQL 4.1.1. See Section 5.1.7, “SQL Modes”.
sql_slave_skip_counter
The number of events from the master that a slave server should skip. See Section 12.6.2.6, “SET GLOBAL SQL_SLAVE_SKIP_COUNTER Syntax”. This variable was added in MySQL 3.23.33.
storage_engine
This variable is a synonym for table_type. It was added in MySQL 4.1.2.
sync_binlog
If the value of this variable is positive, the MySQL server synchronizes its binary log to disk (using fdatasync()) after every sync_binlog writes to the binary log. Note that there is one write to the binary log per statement if autocommit is enabled, and one write per transaction otherwise. The default value is 0, which does no synchronizing to disk. A value of 1 is the safest choice, because in the event of a crash you lose at most one statement or transaction from the binary log. However, it is also the slowest choice (unless the disk has a battery-backed cache, which makes synchronization very fast). This variable was added in MySQL 4.1.3.
If the value of sync_binlog is 0 (the default), no extra flushing is done. The server relies on the operating system to flush the file contents occasionally as for any other file.
sync_frm
If this variable is set to 1, when any non-temporary table is created its .frm file is synchronized to disk (using fdatasync()). This is slower but safer in case of a crash. The default is 1. This was added as a command-line option in MySQL 4.0.18. It is also a settable global variable as of MySQL 4.1.3.
system_time_zone
The server system time zone. When the server begins executing, it inherits a time zone setting from the machine defaults, possibly modified by the environment of the account used for running the server or the startup script. The value is used to set system_time_zone. Typically the time zone is specified by the TZ environment variable. It also can be specified using the --timezone option of the mysqld_safe script.
The system_time_zone variable differs from time_zone. Although they might have the same value, the latter variable is used to initialize the time zone for each client that connects. See Section 9.7, “MySQL Server Time Zone Support”.
system_time_zone was added in MySQL 4.1.3.
table_cache
The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. You can check whether you need to increase the table cache by checking the Opened_tables status variable. See Section 5.1.6, “Status Variables”. If the value of Opened_tables is large and you do not do FLUSH TABLES often (which just forces all tables to be closed and reopened), then you should increase the value of the table_cache variable. For more information about the table cache, see Section 7.4.8, “How MySQL Opens and Closes Tables”.
table_type
The default table type (storage engine). To set the table type at server startup, use the --default-table-type option. This variable was added in MySQL 3.23.0. See Section 5.1.2, “Command Options”.
thread_cache_size
How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there are fewer than thread_cache_size threads there. Requests for threads are satisfied by reusing threads taken from the cache if possible, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. (Normally, this doesn't provide a notable performance improvement if you have a good thread implementation.) By examining the difference between the Connections and Threads_created status variables, you can see how efficient the thread cache is. For details, see Section 5.1.6, “Status Variables”. This variable was added in MySQL 3.23.16.
thread_concurrency
On Solaris, mysqld calls thr_setconcurrency() with this value. This function enables applications to give the threads system a hint about the desired number of threads that should be run at the same time. This variable was added in MySQL 3.23.7.
thread_stack
The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. The default is large enough for normal operation. See Section 7.1.4, “The MySQL Benchmark Suite”. The default is 64KB before MySQL 4.0.10 and 192KB thereafter. If the thread stack size is too small, it limits the complexity of the SQL statements that the server can handle, the recursion depth of stored procedures, and other memory-consuming actions.
time_format
This variable is unused.
time_zone
The current time zone. This variable is used to initialize the time zone for each client that connects. By default, the initial value of this is 'SYSTEM' (which means, “use the value of system_time_zone”). The value can be specified explicitly at server startup with the --default-time-zone option. See Section 9.7, “MySQL Server Time Zone Support”. This variable was added in MySQL 4.1.3.
timezone
The time zone for the server. This is set from the TZ environment variable when mysqld is started. The time zone also can be set by giving a --timezone argument to mysqld_safe. This variable was added in MySQL 3.23.15. As of MySQL 4.1.3, it is obsolete and has been replaced by the system_time_zone variable. See Section A.1.4.6, “Time Zone Problems”.
tmp_table_size
The maximum size of internal in-memory temporary tables. (The actual limit is determined as the smaller of max_heap_table_size and tmp_table_size.) If an in-memory temporary table exceeds the limit, MySQL automatically converts it to an on-disk MyISAM table. Increase the value of tmp_table_size (and max_heap_table_size if necessary) if you do many advanced GROUP BY queries and you have lots of memory. This variable does not apply to user-created MEMORY tables.
tmpdir
The directory used for temporary files and temporary tables. Starting from MySQL 4.1, this variable can be set to a list of several paths that are used in round-robin fashion. Paths should be separated by colon characters (“:”) on Unix and semicolon characters (“;”) on Windows, NetWare, and OS/2.
The multiple-directory feature can be used to spread the load between several physical disks. If the MySQL server is acting as a replication slave, you should not set tmpdir to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication fails. However, if you are using MySQL 4.0.0 or later, you can set the slave's temporary directory using the slave_load_tmpdir variable. In that case, the slave won't use the general tmpdir value and you can set tmpdir to a non-permanent location.
This variable was added in MySQL 3.22.4.
transaction_alloc_block_size
The amount in bytes by which to increase a per-transaction memory pool which needs memory. See the description of transaction_prealloc_size. This variable was added in MySQL 4.0.16.
transaction_prealloc_size
There is a per-transaction memory pool from which various transaction-related allocations take memory. The initial size of the pool in bytes is transaction_prealloc_size. For every allocation that cannot be satisfied from the pool because it has insufficient memory available, the pool is increased by transaction_alloc_block_size bytes. When the transaction ends, the pool is truncated to transaction_prealloc_size bytes.
By making transaction_prealloc_size sufficiently large to contain all statements within a single transaction, you can avoid many malloc() calls. This variable was added in MySQL 4.0.16.
tx_isolation
The default transaction isolation level. This variable was added in MySQL 4.0.3.
This variable is set by the SET TRANSACTION ISOLATION LEVEL statement. See Section 12.4.6, “SET TRANSACTION Syntax”. If you set tx_isolation directly to an isolation level name that contains a space, the name should be enclosed within quotes, with the space replaced by a dash. For example:
```
SET tx_isolation = 'READ-COMMITTED';
```
Any unique prefix of a valid value may be used to set the value of this variable.
version
The version number for the server.
version_bdb
The BDB storage engine version. This variable was added in MySQL 3.23.31 with the name bdb_version and renamed to version_bdb in MySQL 4.1.1.
version_comment
The configure script has a --with-comment option that allows a comment to be specified when building MySQL. This variable contains the value of that comment. This variable was added in MySQL 4.0.17.
version_compile_machine
The type of machine or architecture on which MySQL was built. This variable was added in MySQL 4.1.1.
version_compile_os
The type of operating system on which MySQL was built. This variable was added in MySQL 4.0.19.
wait_timeout
The number of seconds the server waits for activity on a non-interactive connection before closing it. This timeout applies only to TCP/IP and Unix socket file connections, not to connections made via named pipes, or shared memory.
On thread startup, the session wait_timeout value is initialized from the global wait_timeout value or from the global interactive_timeout value, depending on the type of client (as defined by the CLIENT_INTERACTIVE connect option to mysql_real_connect()). See also interactive_timeout.

MySQL Enterprise Expert use of server system variables is part of the service offered by the MySQL Enterprise Monitor. To subscribe, see http://www.mysql.com/products/enterprise/advisors.html.

5.1.4. Session System Variables

Several system variables exist only as session variables. These cannot be set at server startup but can be assigned values at runtime using the SET statement (except for those that are read only). Most of them are not displayed by SHOW VARIABLES, but you can obtain their values using SELECT. This section describes the session system variables. For information about setting or displaying their values, see Section 5.1.5, “Using System Variables”. For example:

mysql> SELECT @@AUTOCOMMIT;
+--------------+
| @@AUTOCOMMIT |
+--------------+
|            1 |
+--------------+

The lettercase of these variables does not matter.

The following table lists the system variables that have only session scope:

Name	Cmd-Line	Option file	System Var	Dynamic
autocommit			Yes	Yes
big-tables	Yes	Yes
- Variable: big_tables			Yes	Yes
error_count			Yes	No
foreign_key_checks			Yes	Yes
identity			Yes	Yes
insert_id			Yes	Yes
last_insert_id			Yes	Yes
sql_auto_is_null			Yes	Yes
sql_big_tables			Yes	Yes
sql_buffer_result			Yes	Yes
sql_log_bin			Yes	Yes
sql_log_off			Yes	Yes
sql_log_update			Yes	Yes
sql_notes			Yes	Yes
sql_quote_show_create			Yes	Yes
sql_safe_updates			Yes	Yes
sql_select_limit	Yes	Yes	Yes	Yes
sql_warnings			Yes	Yes
timestamp			Yes	Yes
unique_checks			Yes	Yes
warning_count			Yes	No

AUTOCOMMIT = {0 | 1}
Set the autocommit mode. If set to 1, all changes to a table take effect immediately. If set to 0 you have to use COMMIT to accept a transaction or ROLLBACK to cancel it. By default, client connections begin with AUTOCOMMIT set to 1. If you change AUTOCOMMIT mode from 0 to 1, MySQL performs an automatic COMMIT of any open transaction. Another way to begin a transaction is to use a START TRANSACTION or BEGIN statement. See Section 12.4.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.
BIG_TABLES = {0 | 1}
If set to 1, all temporary tables are stored on disk rather than in memory. This is a little slower, but the error The table tbl_name is full does not occur for SELECT operations that require a large temporary table. The default value for a new connection is 0 (use in-memory temporary tables). As of MySQL 4.0, you should normally never need to set this variable, because MySQL automatically converts in-memory tables to disk-based tables as necessary.
Note
This variable previously was named SQL_BIG_TABLES.
ERROR_COUNT
The number of errors that resulted from the last statement that generated messages. This variable is read only. See Section 12.5.4.9, “SHOW ERRORS Syntax”.
This variable was added in MySQL 4.1.0.
FOREIGN_KEY_CHECKS = {0 | 1}
If set to 1 (the default), foreign key constraints for InnoDB tables are checked. If set to 0, they are ignored. Disabling foreign key checking can be useful for reloading InnoDB tables in an order different from that required by their parent/child relationships. This variable was added in MySQL 3.23.52. See Section 13.2.7.4, “FOREIGN KEY Constraints”.
Setting FOREIGN_KEY_CHECKS to 0 also affects data definition statements: DROP DATABASE drops a database even if it contains tables that have foreign keys that are referred to by tables outside the database, and DROP TABLE drops tables that have foreign keys that are referred to by other tables.
Note
Setting FOREIGN_KEY_CHECKS to 1 does not trigger a scan of the existing table data. Therefore, rows added to the table while FOREIGN_KEY_CHECKS=0 will not be verified for consistency.
IDENTITY = value
This variable is a synonym for the LAST_INSERT_ID variable. It exists for compatibility with other database systems. As of MySQL 3.23.25, you can read its value with SELECT @@IDENTITY. As of MySQL 4.0.3, you can also set its value with SET IDENTITY.
INSERT_ID = value
Set the value to be used by the following INSERT or ALTER TABLE statement when inserting an AUTO_INCREMENT value. This is mainly used with the binary log.
LAST_INSERT_ID = value
Set the value to be returned from LAST_INSERT_ID(). This is stored in the binary log when you use LAST_INSERT_ID() in a statement that updates a table. Setting this variable does not update the value returned by the mysql_insert_id() C API function.
SQL_AUTO_IS_NULL = {0 | 1}
If set to 1 (the default), you can find the last inserted row for a table that contains an AUTO_INCREMENT column by using the following construct:
```
WHERE auto_increment_column IS NULL
```
This behavior is used by some ODBC programs, such as Access.
SQL_BIG_SELECTS = {0 | 1}
If set to 0, MySQL aborts SELECT statements that are likely to take a very long time to execute (that is, statements for which the optimizer estimates that the number of examined rows exceeds the value of max_join_size). This is useful when an inadvisable WHERE statement has been issued. The default value for a new connection is 1, which allows all SELECT statements.
If you set the max_join_size system variable to a value other than DEFAULT, SQL_BIG_SELECTS is set to 0.
SQL_BUFFER_RESULT = {0 | 1}
If set to 1, SQL_BUFFER_RESULT forces results from SELECT statements to be put into temporary tables. This helps MySQL free the table locks early and can be beneficial in cases where it takes a long time to send results to the client. The default value is 0. This variable was added in MySQL 3.23.13.
SQL_LOG_BIN = {0 | 1}
If set to 0, no logging is done to the binary log for the client. The client must have the SUPER privilege to set this option. The default value is 1. This variable was added in MySQL 3.23.16.
SQL_LOG_OFF = {0 | 1}
If set to 1, no logging is done to the general query log for this client. The client must have the SUPER privilege to set this option. The default value is 0.
SQL_LOG_UPDATE = {0 | 1}
If set to 0, no logging is done to the update log for the client. The client must have the SUPER privilege to set this option. The default value is 1. This variable was added in MySQL 3.22.5.
SQL_NOTES = {0 | 1}
If set to 1 (the default), warnings of Note level are recorded. If set to 0, Note warnings are suppressed. mysqldump includes output to set this variable to 0 so that reloading the dump file does not produce warnings for events that do not affect the integrity of the reload operation. SQL_NOTES was added in MySQL 4.1.11.
SQL_QUOTE_SHOW_CREATE = {0 | 1}
If set to 1 (the default), the server quotes identifiers for SHOW CREATE TABLE and SHOW CREATE DATABASE statements. If set to 0, quoting is disabled. This option is enabled by default so that replication works for identifiers that require quoting. See Section 12.5.4.5, “SHOW CREATE TABLE Syntax”, and Section 12.5.4.4, “SHOW CREATE DATABASE Syntax”. This variable was added in MySQL 3.23.26.
SQL_SAFE_UPDATES = {0 | 1}
If set to 1, MySQL aborts UPDATE or DELETE statements that do not use a key in the WHERE clause or a LIMIT clause. This makes it possible to catch UPDATE or DELETE statements where keys are not used properly and that would probably change or delete a large number of rows. The default value is 0. This variable was added in MySQL 3.22.32.
SQL_SELECT_LIMIT = {value | DEFAULT}
The maximum number of rows to return from SELECT statements. The default value for a new connection is the maximum number of rows that the server allows per table, which depends on the server configuration and may be affected if the server build was configured with --with-big-tables. Typical default values are (2³²)–1 or (2⁶⁴)–1. If you have changed the limit, the default value can be restored by assigning a value of DEFAULT.
If a SELECT has a LIMIT clause, the LIMIT takes precedence over the value of SQL_SELECT_LIMIT.
SQL_SELECT_LIMIT does not apply to SELECT statements executed within stored routines. It also does not apply to SELECT statements that do not produce a result set to be returned to the client. These include SELECT statements in subqueries, CREATE TABLE ... SELECT, and INSERT INTO ... SELECT.
SQL_WARNINGS = {0 | 1}
This variable controls whether single-row INSERT statements produce an information string if warnings occur. The default is 0. Set the value to 1 to produce an information string. This variable was added in MySQL 3.22.11.
TIMESTAMP = {timestamp_value | DEFAULT}
Set the time for this client. This is used to get the original timestamp if you use the binary log to restore rows. timestamp_value should be a Unix epoch timestamp, not a MySQL timestamp.
UNIQUE_CHECKS = {0 | 1}
If set to 1 (the default), uniqueness checks for secondary indexes in InnoDB tables are performed. If set to 0, storage engines are allowed to assume that duplicate keys are not present in input data. If you know for certain that your data does not contain uniqueness violations, you can set this to 0 to speed up large table imports to InnoDB. This variable was added in MySQL 3.23.52.
Note that setting this variable to 0 does not require storage engines to ignore duplicate keys. An engine is still allowed to check for them and issue duplicate-key errors if it detects them.
WARNING_COUNT
The number of errors, warnings, and notes that resulted from the last statement that generated messages. This variable is read only. See Section 12.5.4.21, “SHOW WARNINGS Syntax”.
This variable was added in MySQL 4.1.0.

5.1.5. Using System Variables

5.1.5.1. Structured System Variables
5.1.5.2. Dynamic System Variables

The MySQL server maintains many system variables that indicate how it is configured. Section 5.1.3, “System Variables”, describes the meaning of these variables. Each system variable has a default value. System variables can be set at server startup using options on the command line or in an option file. As of MySQL 4.0.3, most of them can be changed dynamically while the server is running by means of the SET statement, which enables you to modify operation of the server without having to stop and restart it. You can refer to system variable values in expressions.

Beginning with MySQL 4.0.3, the server maintains two kinds of system variables. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections. A given system variable can have both a global and a session value. Global and session system variables are related as follows:

When the server starts, it initializes all global variables to their default values. These defaults can be changed by options specified on the command line or in an option file. (See Section 4.2.3, “Specifying Program Options”.)
The server also maintains a set of session variables for each client that connects. The client's session variables are initialized at connect time using the current values of the corresponding global variables. For example, the client's SQL mode is controlled by the session sql_mode value, which is initialized when the client connects to the value of the global sql_mode value.

System variable values can be set globally at server startup by using options on the command line or in an option file. When you use a startup option to set a variable that takes a numeric value, the value can be given with a suffix of K, M, or G (either uppercase or lowercase) to indicate a multiplier of 1024, 1024² or 1024³; that is, units of kilobytes, megabytes, or gigabytes, respectively. Thus, the following command starts the server with a query cache size of 16 megabytes and a maximum packet size of one gigabyte:

mysqld --query_cache_size=16M --max_allowed_packet=1G

Before MySQL 4.0.3, use this syntax instead:

mysqld --set-variable=query_cache_size=16M \
       --set-variable=max_allowed_packet=1G

Within an option file, those variables are set like this:

[mysqld]
query_cache_size=16M
max_allowed_packet=1G

Or like this before MySQL 4.0.2:

[mysqld]
set-variable=query_cache_size=16M
set-variable=max_allowed_packet=1G

The lettercase of suffix letters does not matter; 16M and 16m are equivalent, as are 1G and 1g.

If you want to restrict the maximum value to which a system variable can be set at runtime with the SET statement, you can specify this maximum by using an option of the form --maximum-var_name=value at server startup. For example, to prevent the value of query_cache_size from being increased to more than 32MB at runtime, use the option --maximum-query_cache_size=32M. This feature is available as of MySQL 4.0.2.

Many system variables are dynamic and can be changed while the server runs by using the SET statement. For a list, see Section 5.1.5.2, “Dynamic System Variables”. To change a system variable with SET, refer to it as var_name, optionally preceded by a modifier:

To indicate explicitly that a variable is a global variable, precede its name by GLOBAL or @@global.. The SUPER privilege is required to set global variables.
To indicate explicitly that a variable is a session variable, precede its name by SESSION, @@session., or @@. Setting a session variable requires no special privilege, but a client can change only its own session variables, not those of any other client.
LOCAL and @@local. are synonyms for SESSION and @@session..
If no modifier is present, SET changes the session variable.

A SET statement can contain multiple variable assignments, separated by commas. If you set several system variables, the most recent GLOBAL or SESSION modifier in the statement is used for following variables that have no modifier specified.

Examples:

SET sort_buffer_size=10000;
SET @@local.sort_buffer_size=10000;
SET GLOBAL sort_buffer_size=1000000, SESSION sort_buffer_size=1000000;
SET @@sort_buffer_size=1000000;
SET @@global.sort_buffer_size=1000000, @@local.sort_buffer_size=1000000;

The @@var_name syntax for system variables is supported for compatibility with some other database systems.

If you change a session system variable, the value remains in effect until your session ends or until you change the variable to a different value. The change is not visible to other clients.

If you change a global system variable, the value is remembered and used for new connections until the server restarts. (To make a global system variable setting permanent, you should set it in an option file.) The change is visible to any client that accesses that global variable. However, the change affects the corresponding session variable only for clients that connect after the change. The global variable change does not affect the session variable for any client that is currently connected (not even that of the client that issues the SET GLOBAL statement).

To prevent incorrect usage, MySQL produces an error if you use SET GLOBAL with a variable that can only be used with SET SESSION or if you do not specify GLOBAL (or @@global.) when setting a global variable.

To set a SESSION variable to the GLOBAL value or a GLOBAL value to the compiled-in MySQL default value, use the DEFAULT keyword. For example, the following two statements are identical in setting the session value of max_join_size to the global value:

SET max_join_size=DEFAULT;
SET @@session.max_join_size=@@global.max_join_size;

Not all system variables can be set to DEFAULT. In such cases, use of DEFAULT results in an error.

You can refer to the values of specific global or sesson system variables in expressions by using one of the @@-modifiers. For example, you can retrieve values in a SELECT statement like this:

SELECT @@global.sql_mode, @@session.sql_mode, @@sql_mode;

When you refer to a system variable in an expression as @@var_name (that is, when you do not specify @@global. or @@session.), MySQL returns the session value if it exists and the global value otherwise. (This differs from SET @@var_name = value, which always refers to the session value.)

Suffixes for specifying a value multiplier can be used when setting a variable at server startup, but not to set the value with SET at runtime. On the other hand, with SET you can assign a variable's value using an expression, which is not true when you set a variable at server startup. For example, the first of the following lines is legal at server startup, but the second is not:

shell> mysql --max_allowed_packet=16M
shell> mysql --max_allowed_packet=16*1024*1024

Conversely, the second of the following lines is legal at runtime, but the first is not:

mysql> SET GLOBAL max_allowed_packet=16M;
mysql> SET GLOBAL max_allowed_packet=16*1024*1024;

Note

Some system variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. However, to set such a variable on the command line or in an option file, you must set it to 1 or 0; setting it to ON or OFF will not work. For example, on the command line, --delay_key_write=1 works but --delay_key_write=ON does not.

To display system variable names and values, use the SHOW VARIABLES statement:

mysql> SHOW VARIABLES;
+---------------------------------+-------------------------------------+
| Variable_name                   | Value                               |
+---------------------------------+-------------------------------------+
| back_log                        | 50                                  |
| basedir                         | /usr/local/mysql                    |
| bdb_cache_size                  | 8388600                             |
| bdb_home                        | /usr/local/mysql                    |
| bdb_log_buffer_size             | 32768                               |
| bdb_logdir                      |                                     |
| bdb_max_lock                    | 10000                               |
| bdb_shared_data                 | OFF                                 |
| bdb_tmpdir                      | /tmp/                               |
| binlog_cache_size               | 32768                               |
| bulk_insert_buffer_size         | 8388608                             |
| character_set_client            | latin1                              |
| character_set_connection        | latin1                              |
| character_set_database          | latin1                              |
| character_set_results           | latin1                              |
| character_set_server            | latin1                              |
| character_set_system            | utf8                                |
| character_sets_dir              | /usr/local/mysql/share/charsets/    |
| collation_connection            | latin1_swedish_ci                   |
| collation_database              | latin1_swedish_ci                   |
| collation_server                | latin1_swedish_ci                   |
...
| innodb_additional_mem_pool_size | 1048576                             |
| innodb_autoextend_increment     | 8                                   |
| innodb_buffer_pool_awe_mem_mb   | 0                                   |
| innodb_buffer_pool_size         | 8388608                             |
| innodb_data_file_path           | ibdata1:10M:autoextend              |
| innodb_data_home_dir            |                                     |
...
| version                         | 4.1.18-max-log                      |
| version_comment                 | MySQL Community Edition - Max (GPL) |
| version_compile_machine         | i686                                |
| version_compile_os              | pc-linux-gnu                        |
| wait_timeout                    | 28800                               |
+---------------------------------+-------------------------------------+

With a LIKE clause, the statement displays only those variables that match the pattern. To obtain a specific variable name, use a LIKE clause as shown:

SHOW VARIABLES LIKE 'max_join_size';
SHOW SESSION VARIABLES LIKE 'max_join_size';

To get a list of variables whose name match a pattern, use the “%” wildcard character in a LIKE clause:

SHOW VARIABLES LIKE '%size%';
SHOW GLOBAL VARIABLES LIKE '%size%';

Wildcard characters can be used in any position within the pattern to be matched. Strictly speaking, because “_” is a wildcard that matches any single character, you should escape it as “\_” to match it literally. In practice, this is rarely necessary.

For SHOW VARIABLES, if you specify neither GLOBAL nor SESSION, MySQL returns SESSION values.

The reason for requiring the GLOBAL keyword when setting GLOBAL-only variables but not when retrieving them is to prevent problems in the future. If we were to remove a SESSION variable that has the same name as a GLOBAL variable, a client with the SUPER privilege might accidentally change the GLOBAL variable rather than just the SESSION variable for its own connection. If we add a SESSION variable with the same name as a GLOBAL variable, a client that intends to change the GLOBAL variable might find only its own SESSION variable changed.

5.1.5.1. Structured System Variables

Structured system variables are supported beginning with MySQL 4.1.1. A structured variable differs from a regular system variable in two respects:

Its value is a structure with components that specify server parameters considered to be closely related.
There might be several instances of a given type of structured variable. Each one has a different name and refers to a different resource maintained by the server.

In MySQL 4.1 (4.1.1 and above), MySQL supports one structured variable type. It specifies parameters that govern the operation of key caches. A key cache structured variable has these components:

key_buffer_size
key_cache_block_size
key_cache_division_limit
key_cache_age_threshold

The purpose of this section is to describe the syntax for referring to structured variables. Key cache variables are used for syntax examples, but specific details about how key caches operate are found elsewhere, in Section 7.4.6, “The MyISAM Key Cache”.

To refer to a component of a structured variable instance, you can use a compound name in instance_name.component_name format. Examples:

hot_cache.key_buffer_size
hot_cache.key_cache_block_size
cold_cache.key_cache_block_size

For each structured system variable, an instance with the name of default is always predefined. If you refer to a component of a structured variable without any instance name, the default instance is used. Thus, default.key_buffer_size and key_buffer_size both refer to the same system variable.

Structured variable instances and components follow these naming rules:

For a given type of structured variable, each instance must have a name that is unique within variables of that type. However, instance names need not be unique across structured variable types. For example, each structured variable has an instance named default, so default is not unique across variable types.
The names of the components of each structured variable type must be unique across all system variable names. If this were not true (that is, if two different types of structured variables could share component member names), it would not be clear which default structured variable to use for references to member names that are not qualified by an instance name.
If a structured variable instance name is not legal as an unquoted identifier, refer to it as a quoted identifier using backticks. For example, hot-cache is not legal, but `hot-cache` is.
global, session, and local are not legal instance names. This avoids a conflict with notation such as @@global.var_name for referring to non-structured system variables.

At the moment, the first two rules have no possibility of being violated because the only structured variable type is the one for key caches. These rules will assume greater significance if some other type of structured variable is created in the future.

With one exception, it is allowable to refer to structured variable components using compound names in any context where simple variable names can occur. For example, you can assign a value to a structured variable using a command-line option:

shell> mysqld --hot_cache.key_buffer_size=64K

In an option file, use this syntax:

[mysqld]
hot_cache.key_buffer_size=64K

If you start the server with such an option, it creates a key cache named hot_cache with a size of 64KB in addition to the default key cache that has a default size of 8MB.

Suppose that you start the server as follows:

shell> mysqld --key_buffer_size=256K \
         --extra_cache.key_buffer_size=128K \
         --extra_cache.key_cache_block_size=2048

In this case, the server sets the size of the default key cache to 256KB. (You could also have written --default.key_buffer_size=256K.) In addition, the server creates a second key cache named extra_cache that has a size of 128KB, with the size of block buffers for caching table index blocks set to 2048 bytes.

The following example starts the server with three different key caches having sizes in a 3:1:1 ratio:

shell> mysqld --key_buffer_size=6M \
         --hot_cache.key_buffer_size=2M \
         --cold_cache.key_buffer_size=2M

Structured variable values may be set and retrieved at runtime as well. For example, to set a key cache named hot_cache to a size of 10MB, use either of these statements:

mysql> SET GLOBAL hot_cache.key_buffer_size = 10*1024*1024;
mysql> SET @@global.hot_cache.key_buffer_size = 10*1024*1024;

To retrieve the cache size, do this:

mysql> SELECT @@global.hot_cache.key_buffer_size;

However, the following statement does not work. The variable is not interpreted as a compound name, but as a simple string for a LIKE pattern-matching operation:

mysql> SHOW GLOBAL VARIABLES LIKE 'hot_cache.key_buffer_size';

This is the exception to being able to use structured variable names anywhere a simple variable name may occur.

5.1.5.2. Dynamic System Variables

Beginning with MySQL 4.0.3, many server system variables are dynamic and can be set at runtime using SET GLOBAL or SET SESSION. You can also select their values using SELECT. See Section 5.1.5, “Using System Variables”.

The following table shows the full list of all dynamic system variables. The last column indicates for each variable whether GLOBAL or SESSION (or both) apply. The table also lists session options that can be set with the SET statement. Section 12.5.3, “SET Syntax”, discusses these options.

Variables that have a type of “string” take a string value. Variables that have a type of “numeric” take a numeric value. Variables that have a type of “boolean” can be set to 0, 1, ON or OFF. (If you set them on the command line or in an option file, use the numeric values.) Variables that are marked as “enumeration” normally should be set to one of the available values for the variable, but can also be set to the number that corresponds to the desired enumeration value. For enumerated system variables, the first enumeration value corresponds to 0. This differs from ENUM columns, for which the first enumeration value corresponds to 1.

Variable Name	Variable Type
`autocommit`	boolean
`big_tables`	boolean
`binlog_cache_size`	numeric
`bulk_insert_buffer_size`	numeric
`character_set_client`	string
`character_set_connection`	string
`character_set_database`	string
`character_set_results`	string
`character_set_server`	string
`collation_connection`	string
`collation_database`	string
`collation_server`	string
`concurrent_insert`	boolean
`connect_timeout`	numeric
`date_format`	string
`datetime_format`	string
`debug`	string
`default_week_format`	numeric
`delayed_insert_limit`	numeric
`delayed_insert_timeout`	numeric
`delayed_queue_size`	numeric
`delay_key_write`	enumeration
`expire_logs_days`	numeric
`flush`	boolean
`flush_time`	numeric
`foreign_key_checks`	boolean
`ft_boolean_syntax`	string
`group_concat_max_len`	numeric
`identity`	numeric
`init_connect`	string
`init_slave`	string
`innodb_autoextend_increment`	numeric
`innodb_fast_shutdown`	boolean
`innodb_flush_log_at_trx_commit`	numeric
`innodb_max_dirty_pages_pct`	numeric
`innodb_max_purge_lag`	numeric
`innodb_table_locks`	boolean
`innodb_thread_concurrency`	numeric
`insert_id`	numeric
`interactive_timeout`	numeric
`join_buffer_size`	numeric
`key_buffer_size`	numeric
`key_cache_age_threshold`	numeric
`key_cache_block_size`	numeric
`key_cache_division_limit`	numeric
`last_insert_id`	numeric
`lc_time_names`	string
`local_infile`
`log_queries_not_using_indexes`	boolean
`log-warnings`	numeric
`long_query_time`	numeric
`low_priority_updates`	boolean
`max_allowed_packet`	numeric
`max_binlog_cache_size`	numeric
`max_binlog_size`	numeric
`max_connect_errors`	numeric
`max_connections`	numeric
`max_delayed_threads`	numeric
`max_error_count`	numeric
`max_heap_table_size`	numeric
`max_insert_delayed_threads`	numeric
`max_join_size`	numeric
`max_length_for_sort_data`	numeric
`max_prepared_stmt_count`	numeric
`max_relay_log_size`	numeric
`max_seeks_for_key`	numeric
`max_sort_length`	numeric
`max_tmp_tables`	numeric
`max_user_connections`	numeric
`max_write_lock_count`	numeric
`myisam_block_size`	numeric
`myisam_data_pointer_size`	numeric
`myisam_max_sort_file_size`	numeric
`myisam_repair_threads`	numeric
`myisam_sort_buffer_size`	numeric
`myisam_stats_method`	enumeration
`ndb_autoincrement_prefetch_sz`	numeric
`ndb_cache_check_time`	numeric
`ndbcluster`	boolean
`ndb_force_send`	boolean
`ndb_use_exact_count`	boolean
`net_buffer_length`	numeric
`net_read_timeout`	numeric
`net_retry_count`	numeric
`net_write_timeout`	numeric
`new`	boolean
`old_passwords`	boolean
`preload_buffer_size`	numeric
`query_alloc_block_size`	numeric
`query_cache_limit`	numeric
`query_cache_min_res_unit`	numeric
`query_cache_size`	numeric
`query_cache_type`	enumeration
`query_cache_wlock_invalidate`	boolean
`query_prealloc_size`	numeric
`range_alloc_block_size`	numeric
`read_buffer_size`	numeric
`read_only`	numeric
`read_rnd_buffer_size`	numeric
`relay_log_purge`	boolean
`rpl_recovery_rank`	numeric
`safe_show_database`	boolean
`secure_auth`	boolean
`server_id`	numeric
`slave_compressed_protocol`	boolean
`slave_net_timeout`	numeric
`slave_transaction_retries`	numeric
`slow_launch_time`	numeric
`sort_buffer_size`	numeric
`sql_auto_is_null`	boolean
`sql_big_selects`	boolean
`sql_big_tables`	boolean
`sql_buffer_result`	boolean
`sql_log_bin`	boolean
`sql_log_off`	boolean
`sql_log_update`	boolean
`sql_low_priority_updates`	boolean
`sql_max_join_size`	numeric
`sql_mode`	enumeration
`sql_notes`	boolean
`sql_quote_show_create`	boolean
`sql_safe_updates`	boolean
`sql_select_limit`	numeric
`sql_slave_skip_counter`	numeric
`sql_warnings`	boolean
`storage_engine`	enumeration
`sync_binlog`	numeric
`sync_frm`	boolean
`table_cache`	numeric
`table_type`	enumeration
`thread_cache_size`	numeric
`time_format`	string
`timestamp`	string
`time_zone`	string
`tmp_table_size`	numeric
`transaction_alloc_block_size`	numeric
`transaction_prealloc_size`	numeric
`tx_isolation`	enumeration
`unique_checks`	boolean
`wait_timeout`	numeric

MySQL Enterprise Improper configuration of system variables can adversely affect performance and security. The MySQL Enterprise Monitor continually monitors system variables and provides expert advice about appropriate settings. For more information see http://www.mysql.com/products/enterprise/advisors.html.

5.1.6. Status Variables

The server maintains many status variables that provide information about its operation. You can view these variables and their values by using the SHOW STATUS statement (see Section 12.5.4.17, “SHOW STATUS Syntax”).

mysql> SHOW STATUS;
+--------------------------+------------+
| Variable_name            | Value      |
+--------------------------+------------+
| Aborted_clients          | 0          |
| Aborted_connects         | 0          |
| Bytes_received           | 155372598  |
| Bytes_sent               | 1176560426 |
| Connections              | 30023      |
...

The following table lists all available server status variables:

Variable Name	Variable Type
`Aborted_clients`	numeric
`Aborted_connects`	numeric
`Binlog_cache_disk_use`	numeric
`Binlog_cache_use`	numeric
`Bytes_received`	numeric
`Bytes_sent`	numeric
`Com_admin_commands`	numeric
`Com_alter_db`	numeric
`Com_alter_table`	numeric
`Com_analyze`	numeric
`Com_backup_table`	numeric
`Com_begin`	numeric
`Com_change_db`	numeric
`Com_change_master`	numeric
`Com_check`	numeric
`Com_checksum`	numeric
`Com_commit`	numeric
`Com_create_db`	numeric
`Com_create_index`	numeric
`Com_create_table`	numeric
`Com_dealloc_sql`	numeric
`Com_delete`	numeric
`Com_delete_multi`	numeric
`Com_do`	numeric
`Com_drop_db`	numeric
`Com_drop_index`	numeric
`Com_drop_table`	numeric
`Com_drop_user`	numeric
`Com_execute_sql`	numeric
`Com_flush`	numeric
`Com_grant`	numeric
`Com_ha_close`	numeric
`Com_ha_open`	numeric
`Com_ha_read`	numeric
`Com_help`	numeric
`Com_insert`	numeric
`Com_insert_select`	numeric
`Com_kill`	numeric
`Com_load`	numeric
`Com_load_master_data`	numeric
`Com_load_master_table`	numeric
`Com_lock_tables`	numeric
`Com_optimize`	numeric
`Com_preload_keys`	numeric
`Com_prepare_sql`	numeric
`Com_rename_table`	numeric
`Com_repair`	numeric
`Com_replace`	numeric
`Com_replace_select`	numeric
`Com_reset`	numeric
`Com_restore_table`	numeric
`Com_revoke`	numeric
`Com_revoke_all`	numeric
`Com_rollback`	numeric
`Com_savepoint`	numeric
`Com_select`	numeric
`Com_set_option`	numeric
`Com_show_binlog_events`	numeric
`Com_show_binlogs`	numeric
`Com_show_charsets`	numeric
`Com_show_collations`	numeric
`Com_show_column_types`	numeric
`Com_show_create_db`	numeric
`Com_show_create_event`	numeric
`Com_show_create_table`	numeric
`Com_show_databases`	numeric
`Com_show_engine_logs`	numeric
`Com_show_engine_mutex`	numeric
`Com_show_engine_status`	numeric
`Com_show_errors`	numeric
`Com_show_fields`	numeric
`Com_show_grants`	numeric
`Com_show_innodb_status`	numeric
`Com_show_keys`	numeric
`Com_show_logs`	numeric
`Com_show_master_status`	numeric
`Com_show_ndb_status`	numeric
`Com_show_new_master`	numeric
`Com_show_open_tables`	numeric
`Com_show_privileges`	numeric
`Com_show_processlist`	numeric
`Com_show_slave_hosts`	numeric
`Com_show_slave_status`	numeric
`Com_show_status`	numeric
`Com_show_storage_engines`	numeric
`Com_show_tables`	numeric
`Com_show_variables`	numeric
`Com_show_warnings`	numeric
`Com_slave_start`	numeric
`Com_slave_stop`	numeric
`Com_stmt_close`	numeric
`Com_stmt_execute`	numeric
`Com_stmt_fetch`	numeric
`Com_stmt_prepare`	numeric
`Com_stmt_reset`	numeric
`Com_stmt_send_long_data`	numeric
`Com_truncate`	numeric
`Com_unlock_tables`	numeric
`Com_update`	numeric
`Com_update_multi`	numeric
`Created_tmp_disk_tables`	numeric
`Created_tmp_files`	numeric
`Created_tmp_tables`	numeric
`Handler_commit`	numeric
`Handler_delete`	numeric
`Handler_discover`	numeric
`Handler_read_first`	numeric
`Handler_read_key`	numeric
`Handler_read_next`	numeric
`Handler_read_prev`	numeric
`Handler_read_rnd`	numeric
`Handler_read_rnd_next`	numeric
`Handler_rollback`	numeric
`Handler_update`	numeric
`Handler_write`	numeric
`Key_blocks_not_flushed`	numeric
`Key_blocks_unused`	numeric
`Key_blocks_used`	numeric
`Key_read_requests`	numeric
`Key_reads`	numeric
`Key_write_requests`	numeric
`Key_writes`	numeric
`Max_used_connections`	numeric
`Not_flushed_delayed_rows`	numeric
`Opened_tables`	numeric
`Open_files`	numeric
`Open_streams`	numeric
`Open_tables`	numeric
`prepared_stmt_count`	numeric
`Qcache_free_blocks`	numeric
`Qcache_free_memory`	numeric
`Qcache_hits`	numeric
`Qcache_inserts`	numeric
`Qcache_lowmem_prunes`	numeric
`Qcache_not_cached`	numeric
`Qcache_queries_in_cache`	numeric
`Qcache_total_blocks`	numeric
`Questions`	numeric
`Select_full_join`	numeric
`Select_full_range_join`	numeric
`Select_range`	numeric
`Select_range_check`	numeric
`Select_scan`	numeric
`Slave_open_temp_tables`	numeric
`Slow_launch_threads`	numeric
`Slow_queries`	numeric
`Sort_merge_passes`	numeric
`Sort_range`	numeric
`Sort_rows`	numeric
`Sort_scan`	numeric
`Table_locks_immediate`	numeric
`Table_locks_waited`	numeric
`Threads_cached`	numeric
`Threads_connected`	numeric
`Threads_created`	numeric
`Threads_running`	numeric
`Uptime`	numeric

Many status variables are reset to 0 by the FLUSH STATUS statement.

MySQL Enterprise For expert advice on using status variables, subscribe to the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

The status variables have the following meanings. The Com_xxx statement counter variables were added beginning with MySQL 3.23.47. The Qcache_xxx query cache variables were added beginning with MySQL 4.0.1. Otherwise, variables with no version indicated have been present since at least MySQL 3.22.

For meanings of status variables specific to MySQL Cluster, see Section 15.4.4, “MySQL Cluster Status Variables”.

Aborted_clients
The number of connections that were aborted because the client died without closing the connection properly. See Section A.1.2.11, “Communication Errors and Aborted Connections”.
Aborted_connects
The number of failed attempts to connect to the MySQL server. See Section A.1.2.11, “Communication Errors and Aborted Connections”.
Binlog_cache_disk_use
The number of transactions that used the temporary binary log cache but that exceeded the value of binlog_cache_size and used a temporary file to store statements from the transaction. This variable was added in MySQL 4.1.2.
Binlog_cache_use
The number of transactions that used the temporary binary log cache. This variable was added in MySQL 4.1.2.
Bytes_received
The number of bytes received from all clients. This variable was added in MySQL 3.23.7.
Bytes_sent
The number of bytes sent to all clients. This variable was added in MySQL 3.23.7.
Com_xxx
The Com_xxx statement counter variables were added beginning with MySQL 3.23.47. They indicate the number of times each xxx statement has been executed. There is one status variable for each type of statement. For example, Com_delete and Com_insert count DELETE and INSERT statements, respectively. However, if a query result is returned from query cache, the server increments the Qcache_hits status variable, not Com_select. See Section 7.5.3.4, “Query Cache Status and Maintenance”.
New Com_stmt_xxx status variables have been added in MySQL 4.1.13:
- Com_stmt_prepare
- Com_stmt_execute
- Com_stmt_send_long_data
- Com_stmt_reset
- Com_stmt_close
Those variables stand for prepared statement commands. Their names refer to the COM_xxx command set used in the network layer. In other words, their values increase whenever prepared statement API calls such as mysql_stmt_prepare(), mysql_stmt_execute(), and so forth are executed. However, Com_stmt_prepare, Com_stmt_execute and Com_stmt_close also increase for PREPARE, EXECUTE, or DEALLOCATE PREPARE, respectively. Additionally, the values of the older (available since MySQL 4.1.3) statement counter variables Com_prepare_sql, Com_execute_sql, and Com_dealloc_sql increase for the PREPARE, EXECUTE, and DEALLOCATE PREPARE statements.
All of the Com_stmt_xxx variables are increased even if their argument (a prepared statement) is unknown or an error occurred during execution; in other words, their values correspond to the number of requests issued, not to the number of requests successfully completed.
Connections
The number of connection attempts (successful or not) to the MySQL server.
Created_tmp_disk_tables
The number of temporary tables on disk created automatically by the server while executing statements. This variable was added in MySQL 3.23.24.
Created_tmp_files
How many temporary files mysqld has created. This variable was added in MySQL 3.23.28.
Created_tmp_tables
The number of in-memory temporary tables created automatically by the server while executing statements. If Created_tmp_disk_tables is large, you may want to increase the tmp_table_size value to cause temporary tables to be memory-based instead of disk-based.
Delayed_errors
The number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key).
Delayed_insert_threads
The number of INSERT DELAYED handler threads in use.
Delayed_writes
The number of INSERT DELAYED rows written.
Flush_commands
The number of executed FLUSH statements.
Handler_commit
The number of internal COMMIT statements. This variable was added in MySQL 4.0.2.
Handler_delete
The number of times a row was deleted from a table.
Handler_read_first
The number of times the first entry was read from an index. If this value is high, it suggests that the server is doing a lot of full index scans; for example, SELECT col1 FROM foo, assuming that col1 is indexed.
Handler_read_key
The number of requests to read a row based on a key. If this value is high, it is a good indication that your tables are properly indexed for your queries.
Handler_read_next
The number of requests to read the next row in key order. This value is incremented if you are querying an index column with a range constraint or if you are doing an index scan.
Handler_read_prev
The number of requests to read the previous row in key order. This read method is mainly used to optimize ORDER BY ... DESC. This variable was added in MySQL 3.23.6.
Handler_read_rnd
The number of requests to read a row based on a fixed position. This value is high if you are doing a lot of queries that require sorting of the result. You probably have a lot of queries that require MySQL to scan entire tables or you have joins that don't use keys properly.
Handler_read_rnd_next
The number of requests to read the next row in the data file. This value is high if you are doing a lot of table scans. Generally this suggests that your tables are not properly indexed or that your queries are not written to take advantage of the indexes you have.
Handler_rollback
The number of internal ROLLBACK statements. This variable was added in MySQL 4.0.2.
Handler_update
The number of requests to update a row in a table.
Handler_write
The number of requests to insert a row in a table.
Key_blocks_not_flushed
The number of key blocks in the key cache that have changed but have not yet been flushed to disk. This variable was added in MySQL 4.1.1. It used to be known as Not_flushed_key_blocks.
Key_blocks_unused
The number of unused blocks in the key cache. You can use this value to determine how much of the key cache is in use; see the discussion of key_buffer_size in Section 5.1.3, “System Variables”. This variable was added in MySQL 4.1.2.
Key_blocks_used
The number of used blocks in the key cache. This value is a high-water mark that indicates the maximum number of blocks that have ever been in use at one time.
Key_read_requests
The number of requests to read a key block from the cache.
Key_reads
The number of physical reads of a key block from disk. If Key_reads is large, then your key_buffer_size value is probably too small. The cache miss rate can be calculated as Key_reads/Key_read_requests.
Key_write_requests
The number of requests to write a key block to the cache.
Key_writes
The number of physical writes of a key block to disk.
Max_used_connections
The maximum number of connections that have been in use simultaneously since the server started.
Not_flushed_delayed_rows
The number of rows waiting to be written in INSERT DELAY queues.
Not_flushed_key_blocks
The old name for Key_blocks_not_flushed before MySQL 4.1.1.
Open_files
The number of files that are open.
Open_streams
The number of streams that are open (used mainly for logging).
Open_tables
The number of tables that are open.
Opened_tables
The number of tables that have been opened. If Opened_tables is big, your table_cache value is probably too small.
Prepared_stmt_count
The current number of prepared statements. (The maximum number of statements is given by the max_prepared_stmt_count system variable.) This variable was added in MySQL 4.1.23.
Qcache_free_blocks
The number of free memory blocks in the query cache.
Qcache_free_memory
The amount of free memory for the query cache.
Qcache_hits
The number of query cache hits.
Qcache_inserts
The number of queries added to the query cache.
Qcache_lowmem_prunes
The number of queries that were deleted from the query cache because of low memory.
Qcache_not_cached
The number of non-cached queries (not cacheable, or not cached due to the query_cache_type setting).
Qcache_queries_in_cache
The number of queries registered in the query cache.
Qcache_total_blocks
The total number of blocks in the query cache.
Questions
The number of statements that clients have sent to the server.
Rpl_status
The status of fail-safe replication (not yet implemented).
Select_full_join
The number of joins that perform table scans because they do not use indexes. If this value is not 0, you should carefully check the indexes of your tables. This variable was added in MySQL 3.23.25.
Select_full_range_join
The number of joins that used a range search on a reference table. This variable was added in MySQL 3.23.25.
Select_range
The number of joins that used ranges on the first table. This is normally not critical issue even if the value is quite large. This variable was added in MySQL 3.23.25.
Select_range_check
The number of joins without keys that check for key usage after each row. (If this is not equal to 0, you should very carefully check the indexes of your tables.) This variable was added in MySQL 3.23.25.
Select_scan
The number of joins that did a full scan of the first table. This variable was added in MySQL 3.23.25.
Slave_open_temp_tables
The number of temporary tables that the slave SQL thread currently has open. This variable was added in MySQL 3.23.29.
Slave_retried_transactions
Total (since startup) number of times the replication slave SQL thread has retried transactions. This variable was added in MySQL 4.1.11.
Slave_running
This is ON if this server is a slave that is connected to a master, and both the I/O SQL and threads are running. This variable was added in MySQL 3.23.16.
Slow_launch_threads
The number of threads that have taken more than slow_launch_time seconds to create. This variable was added in MySQL 3.23.15.
Slow_queries
The number of queries that have taken more than long_query_time seconds. See Section 5.3.5, “The Slow Query Log”.
Sort_merge_passes
The number of merge passes that the sort algorithm has had to do. If this value is large, you should consider increasing the value of the sort_buffer_size system variable. This variable was added in MySQL 3.23.28.
Sort_range
The number of sorts that were done with ranges. This variable was added in MySQL 3.23.25.
Sort_rows
The number of sorted rows. This variable was added in MySQL 3.23.25.
Sort_scan
The number of sorts that were done by scanning the table. This variable was added in MySQL 3.23.25.
Ssl_xxx
Variables used for SSL connections. These variables were added in MySQL 4.0.0.
Table_locks_immediate
The number of times that a request for a table lock could be granted immediately. This variable was added in MySQL 3.23.33.
Table_locks_waited
The number of times that a request for a table lock could not be granted immediately and a wait was needed. If this is high and you have performance problems, you should first optimize your queries, and then either split your table or tables or use replication. This variable was added in MySQL 3.23.33.
Threads_cached
The number of threads in the thread cache. This variable was added in MySQL 3.23.17.
Threads_connected
The number of currently open connections.
Threads_created
The number of threads created to handle connections. If Threads_created is big, you may want to increase the thread_cache_size value. The cache miss rate can be calculated as Threads_created divided by Connections. This variable was added in MySQL 3.23.31.
Threads_running
The number of threads that are not sleeping.
Uptime
The number of seconds that the server has been up.

5.1.7. SQL Modes

The MySQL server can operate in different SQL modes, and (as of MySQL 4.1) can apply these modes differentially for different clients. This capability enables each application to tailor the server's operating mode to its own requirements.

Modes define what SQL syntax MySQL should support and what kind of data validation checks it should perform. This makes it easier to use MySQL in different environments and to use MySQL together with other database servers.

You can set the default SQL mode by starting mysqld with the --sql-mode="modes" option, or by using sql-mode="modes" in my.cnf (Unix operating systems) or my.ini (Windows). modes is a list of different modes separated by comma (“,”) characters. The default value is empty (no modes set). The modes value also can be empty (--sql-mode="" on the command line, or sql-mode="" in my.cnf on Unix systems or in my.ini on Windows) if you want to clear it explicitly.

Beginning with MySQL 4.1, you can change the SQL mode at runtime by using a SET [GLOBAL|SESSION] sql_mode='modes' statement to set the sql_mode system value. Setting the GLOBAL variable requires the SUPER privilege and affects the operation of all clients that connect from that time on. Setting the SESSION variable affects only the current client. Any client can change its own session sql_mode value at any time.

You can retrieve the current global or session sql_mode value with the following statements:

SELECT @@global.sql_mode;
SELECT @@session.sql_mode;

This mode changes syntax and behavior to conform more closely to standard SQL, and is available beginning in MySQL 4.1.1.

The following list describes all supported modes:

ANSI_QUOTES
Treat “"” as an identifier quote character (like the “`” quote character) and not as a string quote character. You can still use “`” to quote identifiers with this mode enabled. With ANSI_QUOTES enabled, you cannot use double quotes to quote literal strings, because it is interpreted as an identifier. (Added in MySQL 4.0.0)
IGNORE_SPACE
Allow spaces between a function name and the “(” character. This causes built-in function names to be treated as reserved words. As a result, identifiers that are the same as function names must be quoted as described in Section 8.2, “Database, Table, Index, Column, and Alias Names”. For example, because there is a COUNT() function, the use of count as a table name in the following statement causes an error:
```
mysql> CREATE TABLE count (i INT);
ERROR 1064 (42000): You have an error in your SQL syntax
```
The table name should be quoted:
```
mysql> CREATE TABLE `count` (i INT);
Query OK, 0 rows affected (0.00 sec)
```
The IGNORE_SPACE SQL mode applies to built-in functions, not to user-defined functions. It is always allowable to have spaces after a UDF name, regardless of whether IGNORE_SPACE is enabled.
For further discussion of IGNORE_SPACE, see Section 8.2.3, “Function Name Parsing and Resolution”.
(Added in MySQL 4.0.0)
NO_AUTO_VALUE_ON_ZERO
NO_AUTO_VALUE_ON_ZERO affects handling of AUTO_INCREMENT columns. Normally, you generate the next sequence number for the column by inserting either NULL or 0 into it. NO_AUTO_VALUE_ON_ZERO suppresses this behavior for 0 so that only NULL generates the next sequence number. (Added in MySQL 4.1.1)
This mode can be useful if 0 has been stored in a table's AUTO_INCREMENT column. (Storing 0 is not a recommended practice, by the way.) For example, if you dump the table with mysqldump and then reload it, MySQL normally generates new sequence numbers when it encounters the 0 values, resulting in a table with contents different from the one that was dumped. Enabling NO_AUTO_VALUE_ON_ZERO before reloading the dump file solves this problem. As of MySQL 4.1.1, mysqldump automatically includes a statement in the dump output that enables NO_AUTO_VALUE_ON_ZERO to avoid this problem.
NO_DIR_IN_CREATE
When creating a table, ignore all INDEX DIRECTORY and DATA DIRECTORY directives. This option is useful on slave replication servers. (Added in MySQL 4.0.15)
NO_FIELD_OPTIONS
Do not print MySQL-specific column options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode. (Added in MySQL 4.1.1)
NO_KEY_OPTIONS
Do not print MySQL-specific index options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode. (Added in MySQL 4.1.1)
NO_TABLE_OPTIONS
Do not print MySQL-specific table options (such as ENGINE) in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode. (Added in MySQL 4.1.1)

NO_UNSIGNED_SUBTRACTION

In integer subtraction operations, do not mark the result as UNSIGNED if one of the operands is unsigned. In other words, the result of a subtraction is always signed whenever this mode is in effect, even if one of the operands is unsigned. For example, compare the type of column c2 in table t1 with that of column c2 in table t2:

mysql> SET SQL_MODE='';
mysql> CREATE TABLE test (c1 BIGINT UNSIGNED NOT NULL);
mysql> CREATE TABLE t1 SELECT c1 - 1 AS c2 FROM test;
mysql> DESCRIBE t1;
+-------+---------------------+------+-----+---------+-------+
| Field | Type                | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| c2    | bigint(21) unsigned |      |     | 0       |       |
+-------+---------------------+------+-----+---------+-------+

mysql> SET SQL_MODE='NO_UNSIGNED_SUBTRACTION';
mysql> CREATE TABLE t2 SELECT c1 - 1 AS c2 FROM test;
mysql> DESCRIBE t2;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| c2    | bigint(21) |      |     | 0       |       |
+-------+------------+------+-----+---------+-------+

Note that this means that BIGINT UNSIGNED is not 100% usable in all contexts. See Section 11.9, “Cast Functions and Operators”. (Added in MySQL 4.0.2)

mysql> SET SQL_MODE = '';
mysql> SELECT CAST(0 AS UNSIGNED) - 1;
+-------------------------+
| CAST(0 AS UNSIGNED) - 1 |
+-------------------------+
|    18446744073709551615 |
+-------------------------+

mysql> SET SQL_MODE = 'NO_UNSIGNED_SUBTRACTION';
mysql> SELECT CAST(0 AS UNSIGNED) - 1;
+-------------------------+
| CAST(0 AS UNSIGNED) - 1 |
+-------------------------+
|                      -1 |
+-------------------------+

ONLY_FULL_GROUP_BY
Do not allow queries for which the SELECT list refers to non-aggregated columns that are not named in the GROUP BY clause. (Added in MySQL 4.0.0) The following query is invalid with this mode enabled because address is not named in the GROUP BY clause:
```
SELECT name, address, MAX(age) FROM t GROUP BY name;
```
PIPES_AS_CONCAT
Treat || as a string concatenation operator (same as CONCAT()) rather than as a synonym for OR. (Added in MySQL 4.0.0)
REAL_AS_FLOAT
Treat REAL as a synonym for FLOAT. By default, MySQL treats REAL as a synonym for DOUBLE. (Added in MySQL 4.0.0)

The following special modes are provided as shorthand for combinations of mode values from the preceding list. All are available as of MySQL 4.1.1.

The descriptions include all mode values that are available in the most recent version of MySQL. For older versions, a combination mode does not include individual mode values that are not available except in newer versions.

ANSI
Equivalent to REAL_AS_FLOAT, PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE. Before MySQL 4.1.11, ANSI also includes ONLY_FULL_GROUP_BY. See Section 1.8.3, “Running MySQL in ANSI Mode”.
DB2
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
MAXDB
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
MSSQL
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
MYSQL323
Equivalent to NO_FIELD_OPTIONS.
MYSQL40
Equivalent to NO_FIELD_OPTIONS.
ORACLE
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
POSTGRESQL
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.

5.1.8. Server-Side Help

As of MySQL 4.1, MySQL Server supports a HELP statement that returns online information from the MySQL Reference manual (see Section 12.3.3, “HELP Syntax”). The proper operation of this statement requires that the help tables in the mysql database be initialized with help topic information, which is done by processing the contents of the fill_help_tables.sql script.

For a MySQL binary distribution on Unix, help table setup occurs when you run mysql_install_db. For an RPM distribution on Linux or binary distribution on Windows, help table setup occurs as part of the MySQL installation process.

For a MySQL source distribution, you can find the fill_help_tables.sql file in the scripts directory. To load the file manually, make sure that you have initialized the mysql database by running mysql_install_db, and then process the file with the mysql client as follows:

shell> mysql -u root mysql < fill_help_tables.sql

If you are working with Bazaar and a MySQL development source tree, the tree doesn't contain fill_help_tables.sql. You can download the proper file for your version of MySQL from http://dev.mysql.com/doc/. After downloading and uncompressing the file, process it with mysql as just described.

5.1.9. Server Response to Signals

On Unix, signals can be sent to processes. mysqld responds to signals sent to it as follows:

SIGTERM causes the server to shut down.

SIGHUP causes the server to reload the grant tables and flush the logs (like FLUSH PRIVILEGES and FLUSH LOGS). It also writes a status report to the error log that has this format:

Status information:

Current dir: /var/mysql/data/
Running threads: 0  Stack size: 196608
Current locks:

Key caches:
default
Buffer_size:       8388600
Block_size:           1024
Division_limit:        100
Age_limit:             300
blocks used:             0
not flushed:             0
w_requests:              0
writes:                  0
r_requests:              0
reads:                   0

handler status:
read_key:            0
read_next:           0
read_rnd             0
read_first:          1
write:               0
delete               0
update:              0

Table status:
Opened tables:          5
Open tables:            0
Open files:             7
Open streams:           0

Alarm status:
Active alarms:   1
Max used alarms: 2
Next alarm time: 67

On some Mac OS X 10.3 versions, mysqld ignores SIGHUP and SIGQUIT.

5.1.10. The Shutdown Process

The server shutdown process takes place as follows:

The shutdown process is initiated.
Server shutdown can be initiated several ways. For example, a user with the SHUTDOWN privilege can execute a mysqladmin shutdown command. mysqladmin can be used on any platform supported by MySQL. Other operating system-specific shutdown initiation methods are possible as well: The server shuts down on Unix when it receives a SIGTERM signal. A server running as a service on Windows shuts down when the services manager tells it to. (On Windows, a user with Administrator rights can also shut down the server using NET STOP service_name, where service_name is the name of the MySQL service. By default, this is MySQL.)
The server creates a shutdown thread if necessary.
Depending on how shutdown was initiated, the server might create a thread to handle the shutdown process. If shutdown was requested by a client, a shutdown thread is created. If shutdown is the result of receiving a SIGTERM signal, the signal thread might handle shutdown itself, or it might create a separate thread to do so. If the server tries to create a shutdown thread and cannot (for example, if memory is exhausted), it issues a diagnostic message that appears in the error log:
```
Error: Can't create thread to kill server
```
The server stops accepting new connections.
To prevent new activity from being initiated during shutdown, the server stops accepting new client connections. It does this by closing the network connections to which it normally listens for connections: the TCP/IP port, the Unix socket file, the Windows named pipe, and shared memory on Windows.
The server terminates current activity.
For each thread that is associated with a client connection, the connection to the client is broken and the thread is marked as killed. Threads die when they notice that they are so marked. Threads for idle connections die quickly. Threads that currently are processing statements check their state periodically and take longer to die. For additional information about thread termination, see Section 12.5.5.3, “KILL Syntax”, in particular for the instructions about killed REPAIR TABLE or OPTIMIZE TABLE operations on MyISAM tables.
For threads that have an open transaction, the transaction is rolled back. Note that if a thread is updating a non-transactional table, an operation such as a multiple-row UPDATE or INSERT may leave the table partially updated, because the operation can terminate before completion.
If the server is a master replication server, threads associated with currently connected slaves are treated like other client threads. That is, each one is marked as killed and exits when it next checks its state.
If the server is a slave replication server, the I/O and SQL threads, if active, are stopped before client threads are marked as killed. The SQL thread is allowed to finish its current statement (to avoid causing replication problems), and then stops. If the SQL thread was in the middle of a transaction at this point, the transaction is rolled back.
Storage engines are shut down or closed.
At this stage, the table cache is flushed and all open tables are closed.
Each storage engine performs any actions necessary for tables that it manages. For example, MyISAM flushes any pending index writes for a table. InnoDB flushes its buffer pool to disk, writes the current LSN to the tablespace, and terminates its own internal threads.
The server exits.

5.2. The mysqld-max Extended MySQL Server

A MySQL-Max server is a version of the mysqld MySQL server that has been built to include additional features. The MySQL-Max distribution to use depends on your platform:

For Windows, MySQL binary distributions include both the standard server (mysqld.exe) and the MySQL-Max server (mysqld-max.exe), so no special distribution is needed. Just use a regular Windows distribution. See Section 2.3, “Installing MySQL on Windows”.
For Linux, if you install MySQL using RPM distributions, the MySQL-Max RPM presupposes that you have already installed the regular server RPM. Use the regular MySQL-server RPM first to install a standard server named mysqld, and then use the MySQL-Max RPM to install a server named mysqld-max. See Section 2.4, “Installing MySQL from RPM Packages on Linux”, for more information on the Linux RPM packages.
All other MySQL-Max distributions contain a single server that is named mysqld but that has the additional features included.

You can find the MySQL-Max binaries on the MySQL AB Web site at http://dev.mysql.com/downloads/.

MySQL AB builds the MySQL-Max servers by using the following configure options:

--with-server-suffix=-max
This option adds a -max suffix to the mysqld version string.
--with-innodb
This option enables support for the InnoDB storage engine. MySQL-Max servers always include InnoDB support, but this option actually is needed only for MySQL 3.23. From MySQL 4.0 onward, InnoDB is included by default in all binary distributions, so a MySQL-Max server is not needed to obtain InnoDB support.
--with-bdb
This option enables support for the Berkeley DB (BDB) storage engine on those platforms for which BDB is available. (See notes in the following discussion.)
--with-blackhole-storage-engine
This option enables support for the BLACKHOLE storage engine in MySQL 4.1.11 and newer.
--with-example-storage-engine
This option enables support for the EXAMPLE storage engine in MySQL 4.1.10 and newer.
--with-ndbcluster
As of MySQL 4.1.2, this option enables support for the NDBCLUSTER storage engine on those platforms for which Cluster is available. (See notes in the following discussion.)
USE_SYMDIR
This define is enabled to turn on database symbolic link support for Windows. This applies only before MySQL 4.0. From MySQL 4.0 onward, symbolic link support is enabled for all Windows servers, so a MySQL-Max server is not needed to take advantage of this feature.

MySQL-Max binary distributions are a convenience for those who wish to install precompiled programs. If you build MySQL using a source distribution, you can build your own Max-like server by enabling the same features at configuration time that the MySQL-Max binary distributions are built with.

MySQL-Max servers include the BerkeleyDB (BDB) storage engine whenever possible, but not all platforms support BDB.

The following table shows on which platforms MySQL-Max binaries include support for BDB and NDB Cluster:

As of MySQL 4.1.2, MySQL Cluster is supported on Linux (on most platforms), Solaris, Mac OS X, and HP-UX only. Some users have reported success in using MySQL Cluster built from source on BSD operating systems, but these are not officially supported at this time. Note that, even for servers compiled with Cluster support, the NDBCLUSTER storage engine is not enabled by default. You must start the server with the --ndbcluster option to use it as part of a MySQL Cluster. (For details, see Section 15.3, “MySQL Cluster Configuration”.)

The following table shows the platforms for which MySQL-Max binaries include support for BDB and NDBCLUSTER.

System	BDB Support	NDB Support
AIX 5.2	N	N
HP-UX	Y	Y
Linux-Alpha	N	N
Linux-IA-64	N	Y
Linux-Intel	Y	Y
Mac OS X	N	Y
NetWare	N	N
SCO 6	N	N
Solaris-SPARC	Y	Y
Solaris-Intel	N	Y
Solaris-AMD 64	Y	Y
Windows NT/2000/XP	Y	N

To find out which storage engines your server supports, use the SHOW ENGINES statement. (See Section 12.5.4.8, “SHOW ENGINES Syntax”.) For example:

mysql> SHOW ENGINES\G
*************************** 1. row ***************************
 Engine: MyISAM
Support: DEFAULT
Comment: Default engine as of MySQL 3.23 with great performance
*************************** 2. row ***************************
 Engine: HEAP
Support: YES
Comment: Alias for MEMORY
*************************** 3. row ***************************
 Engine: MEMORY
Support: YES
Comment: Hash based, stored in memory, useful for temporary tables
*************************** 4. row ***************************
 Engine: MERGE
Support: YES
Comment: Collection of identical MyISAM tables
...

Before MySQL 4.1.2, SHOW ENGINES is unavailable. Use the following statement instead and check the value of the variable for the storage engine in which you are interested:

mysql> SHOW VARIABLES LIKE 'have%';
+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| have_archive          | YES   |
| have_bdb              | YES   |
| have_blackhole_engine | YES   |
| have_compress         | YES   |
| have_crypt            | YES   |
| have_csv              | YES   |
| have_example_engine   | YES   |
| have_geometry         | YES   |
| have_innodb           | YES   |
| have_isam             | NO    |
| have_ndbcluster       | NO    |
| have_openssl          | YES   |
| have_query_cache      | YES   |
| have_raid             | NO    |
| have_rtree_keys       | YES   |
| have_symlink          | YES   |
+-----------------------+-------+
16 rows in set (0.00 sec)

The precise output from these statements may vary according to the MySQL version used (and the features that are enabled). The values of the second column of the output indicate the server's level of support for each feature, as shown here:

Value	Meaning
`YES`	The feature is supported and is active.
`NO`	The feature is not supported.
`DISABLED`	The feature is supported but has been disabled.

A value of NO means that the server was compiled without support for the feature, so it cannot be activated at runtime.

A value of DISABLED occurs either because the server was started with an option that disables the feature, or because not all options required to enable it were given. In the latter case, the error log file should contain a reason indicating why the option is disabled. See Section 5.3.1, “The Error Log”.

One situation in which you might see DISABLED occurs with MySQL 3.23 when the InnoDB storage engine is compiled in. In MySQL 3.23, you must supply at least the innodb_data_file_path option at runtime to set up the InnoDB tablespace. Without this option, InnoDB disables itself. See Section 13.2.3, “InnoDB in MySQL 3.23”. You can specify configuration options for the BDB storage engine, too, but BDB does not disable itself if you do not provide them. See Section 13.5.3, “BDB Startup Options”.

You might also see DISABLED for a storage engine if the server was compiled to support it, but was started with a --skip-engine_name option. For example, --skip-innodb disables the InnoDB engine. For the NDB Cluster storage engine, DISABLED means the server was compiled with support for MySQL Cluster, but was not started with the --ndb-cluster option.

As of version 3.23, all MySQL servers support MyISAM tables, because MyISAM is the default storage engine.

5.3. MySQL Server Logs

5.3.1. The Error Log
5.3.2. The General Query Log
5.3.3. The Update Log
5.3.4. The Binary Log
5.3.5. The Slow Query Log
5.3.6. Server Log Maintenance

MySQL has several different logs that can help you find out what is going on inside mysqld:

Log Type	Information Written to Log
The error log	Problems encountered starting, running, or stopping mysqld
The isam log	All changes to the `ISAM` tables (used only for debugging the `ISAM` code)
The general query log	Established client connections and statements received from clients
The update log	All statements that change data (this log is deprecated)
The binary log	All statements that change data (also used for replication)
The slow query log	All queries that took more than `long_query_time` seconds to execute or didn't use indexes

By default, all log files are created in the mysqld data directory. You can force mysqld to close and reopen the log files (or in some cases switch to a new log) by flushing the logs. Log flushing occurs when you issue a FLUSH LOGS statement or execute mysqladmin flush-logs or mysqladmin refresh. See Section 12.5.5.2, “FLUSH Syntax”, and Section 4.5.2, “mysqladmin — Client for Administering a MySQL Server”.

If you are using MySQL replication capabilities, slave replication servers maintain additional log files called relay logs. Chapter 14, Replication, discusses relay log contents and configuration.

MySQL Enterprise The MySQL Enterprise Monitor provides a number of advisors specifically related to the various log files. For more information see http://www.mysql.com/products/enterprise/advisors.html.

5.3.1. The Error Log

The error log contains information indicating when mysqld was started and stopped and also any critical errors that occur while the server is running. If mysqld notices a table that needs to be automatically checked or repaired, it writes a message to the error log.

On some operating systems, the error log contains a stack trace if mysqld dies. The trace can be used to determine where mysqld died. See MySQL Internals: Porting.

Beginning with MySQL 4.0.10, you can specify where mysqld writes the error log with the --log-error[=file_name] option. If no file_name value is given, mysqld uses the name host_name.err by default and writes the file in the data directory. (Prior to MySQL 4.0.10, the Windows error log name is mysql.err.) If you execute FLUSH LOGS, the error log is renamed with a suffix of -old and mysqld creates a new empty log file. (No renaming occurs if the --log-error option was not given to mysqld.)

In older MySQL versions on Unix, error log handling was done by mysqld_safe which redirected the error file to host_name.err. You could change this filename by specifying a --err-log=file_name option to mysqld_safe.

If you do not specify --log-error, or (on Windows) if you use the --console option, errors are written to stderr, the standard error output. Usually this is your terminal.

On Windows, error output is always written to the .err file if --console is not given.

The --log-warnings option or log_warnings system variable can be used to control warning logging to the error log. The default value is enabled (1) as of MySQL 4.0.19 and 4.1.2. Warning logging can be disabled using a value of 0. As of MySQL 4.0.21 and 4.1.3, the value can be greater than 1. If the value is greater than 1, aborted connections are written to the error log. See Section A.1.2.11, “Communication Errors and Aborted Connections”.

If mysqld_safe is used to start mysqld and mysqld dies unexpectedly, mysqld_safe notices that it needs to restart mysqld and writes a restarted mysqld message to the error log.

5.3.2. The General Query Log

The general query log is a general record of what mysqld is doing. The server writes information to this log when clients connect or disconnect, and it logs each SQL statement received from clients. The general query log can be very useful when you suspect an error in a client and want to know exactly what the client sent to mysqld.

Older versions of the mysql.server script (from MySQL 3.23.4 to 3.23.8) pass a --log option to safe_mysqld to enable the general query log. If you need better performance when you start using MySQL in a production environment, you can remove the --log option from mysql.server or change it to --log-bin. See Section 5.3.4, “The Binary Log”.

mysqld writes statements to the query log in the order that it receives them, which might differ from the order in which they are executed. This logging order contrasts to the update log and the binary log, which are written after the query is executed but before any locks are released. (Also, the query log contains all statements, whereas the update and binary logs do not contain statements that only select data.)

To enable the general query log, start mysqld with the --log[=file_name] or -l [file_name] option.

If no file_name value is given for --log or -l, the default name is host_name.log in the data directory.

Server restarts and log flushing do not cause a new general query log file to be generated (although flushing closes and reopens it). On Unix, you can rename the file and create a new one by using the following commands:

shell> mv host_name.log host_name-old.log
shell> mysqladmin flush-logs
shell> cp host_name-old.log backup-directory
shell> rm host_name-old.log

On Windows, you cannot rename the log file while the server has it open. You must stop the server and rename the file, and then restart the server to create a new log file.

The session sql_log_off variable can be set to ON or OFF to disable or enable general query logging for the current connection.

5.3.3. The Update Log

Note

The update log has been deprecated and replaced by the more useful, informative, and efficient binary log. See Section 5.3.4, “The Binary Log”.

When started with the --log-update[=file_name] option, mysqld writes a log file containing all SQL statements that update data. If no file_name value is given, the default name is name of the host machine. If a filename is given, but it does not contain a leading path, the file is written in the data directory. If file_name does not have an extension, mysqld creates log files with names of the form file_name.nnnnnn, where nnnnnn is a number that is incremented each time you start the server or flush the logs.

Note

For this naming scheme to work, you must not create your own files with the same names as those that might be used in the log file sequence.

Update logging is “smart” in that it logs only statements that actually update data. Thus, an UPDATE or DELETE with a WHERE clause that finds no rows is not written to the log. Update logging also skips UPDATE statements that merely set a column to its existing value.

The update logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that statements are logged in execution order.

If you want to update a database from update log files, you could do the following (assuming that your update logs have names of the form file_name.nnnnnn):

shell> ls -1 -t -r file_name.[0-9]* | xargs cat | mysql

ls is used to sort the update log filenames into the right order.

This can be useful if you have to revert to backup files after a crash and you want to redo the updates that occurred between the time of the backup and the crash.

5.3.4. The Binary Log

The binary log contains all statements that update data or (starting from MySQL 4.1.3) potentially could have updated it (for example, a DELETE which matched no rows). Statements are stored in the form of “events” that describe the modifications. The binary log also contains information about how long each statement took that updated data. The binary log has two important purposes:

For replication, the binary log is used on master replication servers as a record of the statements to be sent to slave servers. The master server sends the events contained in its binary log to its slaves, which execute those events to make the same data changes that were made on the master. See Section 14.2, “Replication Implementation Overview”.
Certain data recovery operations require use of the binary log. After a backup file has been restored, the events in the binary log that were recorded after the backup was made are re-executed. These events bring databases up to date from the point of the backup. See Section 6.2.2, “Using Backups for Recovery”.

Note

The binary log has replaced the old update log, which is being phased out of future MySQL release series after 4.1. The binary log contains all information that is available in the update log in a more efficient format and in a manner that is transaction-safe. If you are using transactions, you must use the MySQL binary log for backups instead of the old update log.

The binary log is not used for statements such as SELECT or SHOW that do not modify data. If you want to log all statements (for example, to identify a problem query), use the general query log. See Section 5.3.2, “The General Query Log”.

MySQL Enterprise The binary log can also be used to track significant DDL events. Analyzing the binary log in this way is an integral part of the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

Running the server with the binary log enabled makes performance about 1% slower. However, the benefits of the binary log for restore operations and in allowing you to set up replication generally outweigh this minor performance decrement.

When started with the --log-bin[=base_name] option, mysqld writes a log file containing all SQL statements that update data (both DDL and DML statements). If no base_name value is given, the default name is the value of the pid-file option (which by default is the name of host machine) followed by -bin. If the basename is given, but not as an absolute pathname, the server writes the file in the data directory. It is recommended that you specify a basename; see Section A.1.8.4, “Open Issues in MySQL”, for the reason.

If you supply an extension in the log name (for example, --log-bin=base_name.extension), the extension is silently removed and ignored.

mysqld appends a numeric extension to the binary log basename to generate binary log filenames. The number increases each time the server creates a new log file, thus creating an ordered series of files. The server creates a new file in the series each time it starts or flushes the logs. The server also creates a new binary log file automatically when the current log's size reaches max_binlog_size. A binary log file may become larger than max_binlog_size if you are using large transactions because a transaction is written to the file in one piece, never split between files.

To keep track of which binary log files have been used, mysqld also creates a binary log index file that contains the names of all used binary log files. By default, this has the same basename as the binary log file, with the extension '.index'. You can change the name of the binary log index file with the --log-bin-index[=file_name] option. You should not manually edit this file while mysqld is running; doing so would confuse mysqld.

You can delete all binary log files with the RESET MASTER statement, or a subset of them with PURGE MASTER LOGS. See Section 12.5.5.5, “RESET Syntax”, and Section 12.6.1.1, “PURGE MASTER LOGS Syntax”.

Before MySQL 4.1.9, writes to a binary log file or binary log index file that failed due to a full disk or an exceeded quota resulted in corruption of the file. Starting from MySQL 4.1.9, writes to the binary log file and binary log index file are handled the same way as writes to MyISAM tables. See Section A.1.4.3, “How MySQL Handles a Full Disk”.

The binary log format has some known limitations that can affect recovery from backups, especially in old versions. These caveats, which also affect replication, are listed at Section 14.7, “Replication Features and Known Problems”. One caveat which does not affect replication but only recovery with mysqlbinlog: before MySQL 4.1, mysqlbinlog could not prepare output suitable for mysql if the binary log contained interlaced statements originating from different clients that used temporary tables of the same name. This is fixed in MySQL 4.1. However, the problem still existed for LOAD DATA INFILE statements until it was fixed in MySQL 4.1.8.

A replication slave server by default does not write to its own binary log any data modifications that are received from the replication master. To log these modifications, start the slave with the --log-slave-updates option.

You can use the following options to mysqld to affect what is logged to the binary log. See also the discussion that follows this option list.

If you are using replication, the options described here affect which statements are sent by a master server to its slaves. There are also options for slave servers that control which statements received from the master to execute or ignore. For details, see Section 14.8, “Replication Startup Options”.

--binlog-do-db=db_name
Tell the server to restrict binary logging to updates for which the default database is db_name (that is, the database selected by USE). All other databases that are not explicitly mentioned are ignored. If you use this option, you should ensure that you do updates only in the default database.
There is an exception to this for CREATE DATABASE, ALTER DATABASE, and DROP DATABASE statements. The server uses the database named in the statement (not the default database) to decide whether it should log the statement.
An example of what does not work as you might expect: If the server is started with binlog-do-db=sales, and you run USE prices; UPDATE sales.january SET amount=amount+1000;, this statement is not written into the binary log.
To log multiple databases, use multiple options, specifying the option once for each database.
--binlog-ignore-db=db_name
Tell the server to suppress binary logging of updates for which the default database is db_name (that is, the database selected by USE). If you use this option, you should ensure that you do updates only in the default database.
As with the --binlog-do-db option, there is an exception for the CREATE DATABASE, ALTER DATABASE, and DROP DATABASE statements. The server uses the database named in the statement (not the default database) to decide whether it should log the statement.
An example of what does not work as you might expect: If the server is started with binlog-ignore-db=sales, and you run USE prices; UPDATE sales.january SET amount=amount+1000;, this statement is written into the binary log.
To ignore multiple databases, use multiple options, specifying the option once for each database.

The server evaluates the options for logging or ignoring updates to the binary log according to the following rules. As described previously, there is an exception for the CREATE DATABASE, ALTER DATABASE, and DROP DATABASE statements. In those cases, the database being created, altered, or dropped replaces the default database in the following rules:

Are there --binlog-do-db or --binlog-ignore-db rules?
- No: Write the statement to the binary log and exit.
- Yes: Go to the next step.
There are some rules (--binlog-do-db, --binlog-ignore-db, or both). Is there a default database (has any database been selected by USE?)?
- No: Do not write the statement, and exit.
- Yes: Go to the next step.
There is a default database. Are there some --binlog-do-db rules?
- Yes: Does the default database match any of the --binlog-do-db rules?
  - Yes: Write the statement and exit.
  - No: Do not write the statement, and exit.
- No: Go to the next step.
There are some --binlog-ignore-db rules. Does the default database match any of the --binlog-ignore-db rules?
- Yes: Do not write the statement, and exit.
- No: Write the query and exit.

For example, a slave running with only --binlog-do-db=sales does not write to the binary log any statement for which the default database is different from sales (in other words, --binlog-do-db can sometimes mean “ignore other databases”).

If you are using replication, you should not delete old binary log files until you are sure that no slave still needs to use them. For example, if your slaves never run more than three days behind, once a day you can execute mysqladmin flush-logs on the master and then remove any logs that are more than three days old. You can remove the files manually, but it is preferable to use PURGE MASTER LOGS, which also safely updates the binary log index file for you (and which can take a date argument as of MySQL 4.1). See Section 12.6.1.1, “PURGE MASTER LOGS Syntax”.

A client that has the SUPER privilege can disable binary logging of its own statements by using a SET SQL_LOG_BIN=0 statement. See Section 12.5.3, “SET Syntax”.

You can display the contents of binary log files with the mysqlbinlog utility. This can be useful when you want to reprocess statements in the log. For example, you can update a MySQL server from the binary log as follows:

shell> mysqlbinlog log_file | mysql -h server_name

See Section 4.6.6, “mysqlbinlog — Utility for Processing Binary Log Files”, for more information on the mysqlbinlog utility and how to use it. mysqlbinlog also can be used with relay log files because they are written using the same format as binary log files.

Binary logging is done immediately after a statement completes but before any locks are released or any commit is done. This ensures that the log is logged in execution order.

Updates to non-transactional tables are stored in the binary log immediately after execution. Within an uncommitted transaction, all updates (UPDATE, DELETE, or INSERT) that change transactional tables such as BDB or InnoDB tables are cached until a COMMIT statement is received by the server. At that point, mysqld writes the entire transaction to the binary log before the COMMIT is executed. When the thread that handles the transaction starts, it allocates a buffer of binlog_cache_size to buffer statements. If a statement is bigger than this, the thread opens a temporary file to store the transaction. The temporary file is deleted when the thread ends.

Modifications to non-transactional tables cannot be rolled back. If a transaction that is rolled back includes modifications to non-transactional tables, the entire transaction is logged with a ROLLBACK statement at the end to ensure that the modifications to those tables are replicated. This is true as of MySQL 4.0.15.

The Binlog_cache_use status variable shows the number of transactions that used this buffer (and possibly a temporary file) for storing statements. The Binlog_cache_disk_use status variable shows how many of those transactions actually had to use a temporary file. These two variables can be used for tuning binlog_cache_size to a large enough value that avoids the use of temporary files.

The max_binlog_cache_size system variable (default 4GB, which is also the maximum) can be used to restrict the total size used to cache a multiple-statement transaction. If a transaction is larger than this many bytes, it fails and rolls back. The minimum value is 4096.

If you are using the update log or binary log, concurrent inserts are converted to normal inserts for CREATE ... SELECT or INSERT ... SELECT statements. This is done to ensure that you can re-create an exact copy of your tables by applying the log during a backup operation.

The binary log format differs between versions 3.23 and 4.0. (These format changes were required to implement enhancements to replication.) However, MySQL 4.1 has the same binary log format as 4.0. See Section 14.5, “Replication Compatibility Between MySQL Versions”.

By default, the binary log is not synchronized to disk at each write. So if the operating system or machine (not only the MySQL server) crashes, there is a chance that the last statements of the binary log are lost. To prevent this, you can make the binary log be synchronized to disk after every N writes to the binary log, with the sync_binlog system variable. See Section 5.1.3, “System Variables”. 1 is the safest value for sync_binlog, but also the slowest. Even with sync_binlog set to 1, there is still the chance of an inconsistency between the table content and binary log content in case of a crash. For example, if you are using InnoDB tables and the MySQL server processes a COMMIT statement, it writes the whole transaction to the binary log and then commits this transaction into InnoDB. If the server crashes between those two operations, the transaction is rolled back by InnoDB at restart but still exists in the binary log. This problem can be solved with the --innodb-safe-binlog option (available starting from MySQL 4.1.3), which adds consistency between the content of InnoDB tables and the binary log.

For this option to provide a greater degree of safety, the MySQL server should also be configured to synchronize the binary log and the InnoDB logs to disk at every transaction. The InnoDB logs are synchronized by default, and sync_binlog=1 can be used to synchronize the binary log. The effect of this option is that at restart after a crash, after doing a rollback of transactions, the MySQL server cuts rolled back InnoDB transactions from the binary log. This ensures that the binary log reflects the exact data of InnoDB tables, and so, that the slave remains in synchrony with the master (not receiving a statement which has been rolled back).

Note that --innodb-safe-binlog can be used even if the MySQL server updates other storage engines than InnoDB. Only statements and transactions that affect InnoDB tables are subject to removal from the binary log at InnoDB's crash recovery. If the MySQL server discovers at crash recovery that the binary log is shorter than it should have been, it lacks at least one successfully committed InnoDB transaction. This should not happen if sync_binlog=1 and the disk/filesystem do an actual sync when they are requested to (some don't), so the server prints an error message The binary log <name> is shorter than its expected size. In this case, this binary log is not correct and replication should be restarted from a fresh snapshot of the master's data.

5.3.5. The Slow Query Log

The slow query log consists of all SQL statements that took more than long_query_time seconds to execute. The time to acquire the initial table locks is not counted as execution time. mysqld writes a statement to the slow query log after it has been executed and after all locks have been released, so log order might be different from execution order. The minimum and default values of long_query_time are 1 and 10, respectively.

To enable the slow query log, start mysqld with the --log-slow-queries[=file_name] option.

If no file_name value is given for --log-slow-queries, the default name is host_name-slow.log. If a filename is given, but not as an absolute pathname, the server writes the file in the data directory.

The slow query log can be used to find queries that take a long time to execute and are therefore candidates for optimization. However, examining a long slow query log can become a difficult task. To make this easier, you can process the slow query log using the mysqldumpslow command to summarize the queries that appear in the log. Use mysqldumpslow --help to see the options that this command supports.

Before MySQL 4.1, if you also use --log-long-format when logging slow queries, queries that are not using indexes are logged as well. Starting with MySQL 4.1, logging of queries not using indexes is enabled using the --log-queries-not-using-indexes option instead. The --log-long-format is deprecated as of MySQL 4.1, when --log-short-format was introduced, which causes less information to be logged. (The long log format is the default setting since version 4.1.). See Section 5.1.2, “Command Options”.

MySQL Enterprise Excessive table scans are indicative of missing or poorly optimized indexes. Using an advisor specifically designed for the task, the MySQL Enterprise Monitor can identify such problems and offer advice on resolution. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

In MySQL 4.0, slow administrative statements such as OPTIMIZE TABLE, ANALYZE TABLE, and ALTER TABLE were written to the slow query log. This logging was disabled in MySQL 4.1 until 4.1.13, when the --log-slow-admin-statements server option was added to specify logging of slow administrative statements.

Queries handled by the query cache are not added to the slow query log, nor are queries that would not benefit from the presence of an index because the table has zero rows or one row.

5.3.6. Server Log Maintenance

MySQL Server can create a number of different log files that make it easy to see what is going on. See Section 5.3, “MySQL Server Logs”. However, you must clean up these files regularly to ensure that the logs do not take up too much disk space.

When using MySQL with logging enabled, you may want to back up and remove old log files from time to time and tell MySQL to start logging to new files. See Section 6.1, “Database Backups”.

On a Linux (Red Hat) installation, you can use the mysql-log-rotate script for this. If you installed MySQL from an RPM distribution, this script should have been installed automatically. You should be careful with this script if you are using the binary log for replication. You should not remove binary logs until you are certain that their contents have been processed by all slaves.

On other systems, you must install a short script yourself that you start from cron (or its equivalent) for handling log files.

For the binary log, you can set the expire_logs_days system variable to expire binary log files automatically after a given number of days (see Section 5.1.3, “System Variables”). If you are using replication, you should set the variable no lower than the maximum number of days your slaves might lag behind the master.

You can force MySQL to start using new log files by issuing a FLUSH LOGS statement or executing mysqladmin flush-logs or mysqladmin refresh. See Section 12.5.5.2, “FLUSH Syntax”, and Section 4.5.2, “mysqladmin — Client for Administering a MySQL Server”. If you are using MySQL 3.21, you must use mysqladmin refresh.

A log flushing operation does the following:

If general query logging (--log) or slow query logging (--log-slow-queries) to a log file is enabled, the server closes and reopens the general query log file or slow query log file.
If update logging (--log-update) or binary logging (--log-bin) is used, closes the log and opens a new log file with a higher sequence number.
If the server was given an error log filename with the --log-error option, it renames the error log with the suffix -old and creates a new empty error log file.

The server creates a new binary log file when you flush the logs. However, it just closes and reopens the general and slow query log files. To cause new files to be created on Unix, rename the current logs before flushing them. At flush time, the server will open new logs with the original names. For example, if the general and slow query logs are named mysql.log and mysql-slow.log, you can use a series of commands like this:

shell> cd mysql-data-directory
shell> mv mysql.log mysql.old
shell> mv mysql-slow.log mysql-slow.old
shell> mysqladmin flush-logs

At this point, you can make a backup of mysql.old and mysql-slow.log and then remove them from disk.

On Windows, you cannot rename log files while the server has them open. You must stop the server and rename them, and then restart the server to create new logs.

5.4. General Security Issues

5.4.1. General Security Guidelines
5.4.2. Making MySQL Secure Against Attackers
5.4.3. Security-Related mysqld Options
5.4.4. Security Issues with LOAD DATA LOCAL
5.4.5. How to Run MySQL as a Normal User

This section describes some general security issues to be aware of and what you can do to make your MySQL installation more secure against attack or misuse. For information specifically about the access control system that MySQL uses for setting up user accounts and checking database access, see Section 5.5, “The MySQL Access Privilege System”.

5.4.1. General Security Guidelines

Anyone using MySQL on a computer connected to the Internet should read this section to avoid the most common security mistakes.

In discussing security, we emphasize the necessity of fully protecting the entire server host (not just the MySQL server) against all types of applicable attacks: eavesdropping, altering, playback, and denial of service. We do not cover all aspects of availability and fault tolerance here.

MySQL uses security based on Access Control Lists (ACLs) for all connections, queries, and other operations that users can attempt to perform. There is also support for SSL-encrypted connections between MySQL clients and servers. Many of the concepts discussed here are not specific to MySQL at all; the same general ideas apply to almost all applications.

When running MySQL, follow these guidelines whenever possible:

Do not ever give anyone (except MySQL root accounts) access to the user table in the mysql database! This is critical, particularly before MySQL 4.1, when the encrypted password is the real password in MySQL: Anyone who knows the password that is listed in the mysql.user table and who has access to the host listed for the account can easily log in as that user. In MySQL 4.1, the password hashing algorithm was changed so that this is no longer true.
Learn the MySQL access privilege system. The GRANT and REVOKE statements are used for controlling access to MySQL. Do not grant more privileges than necessary. Never grant privileges to all hosts.
Checklist:
- Try mysql -u root. If you are able to connect successfully to the server without being asked for a password, anyone can connect to your MySQL server as the MySQL root user with full privileges! Review the MySQL installation instructions, paying particular attention to the information about setting a root password. See Section 2.10.3, “Securing the Initial MySQL Accounts”.
- Use the SHOW GRANTS statement to check which accounts have access to what. Then use the REVOKE statement to remove those privileges that are not necessary.
Do not store any plain-text passwords in your database. If your computer becomes compromised, the intruder can take the full list of passwords and use them. Instead, use MD5(), SHA1(), or some other one-way hashing function and store the hash value.
Do not choose passwords from dictionaries. Special programs exist to break passwords. Even passwords like “xfish98” are very bad. Much better is “duag98” which contains the same word “fish” but typed one key to the left on a standard QWERTY keyboard. Another method is to use a password that is taken from the first characters of each word in a sentence (for example, “Mary had a little lamb” results in a password of “Mhall”). The password is easy to remember and type, but difficult to guess for someone who does not know the sentence.
Invest in a firewall. This protects you from at least 50% of all types of exploits in any software. Put MySQL behind the firewall or in a demilitarized zone (DMZ).
Checklist:
- Try to scan your ports from the Internet using a tool such as nmap. MySQL uses port 3306 by default. This port should not be accessible from untrusted hosts. Another simple way to check whether or not your MySQL port is open is to try the following command from some remote machine, where server_host is the hostname or IP number of the host on which your MySQL server runs:
```
shell> telnet server_host 3306
```
  If you get a connection and some garbage characters, the port is open, and should be closed on your firewall or router, unless you really have a good reason to keep it open. If telnet hangs or the connection is refused, the port is blocked, which is how you want it to be.
Do not trust any data entered by users of your applications. They can try to trick your code by entering special or escaped character sequences in Web forms, URLs, or whatever application you have built. Be sure that your application remains secure if a user enters something like “; DROP DATABASE mysql;”. This is an extreme example, but large security leaks and data loss might occur as a result of hackers using similar techniques, if you do not prepare for them.
A common mistake is to protect only string data values. Remember to check numeric data as well. If an application generates a query such as SELECT * FROM table WHERE ID=234 when a user enters the value 234, the user can enter the value 234 OR 1=1 to cause the application to generate the query SELECT * FROM table WHERE ID=234 OR 1=1. As a result, the server retrieves every row in the table. This exposes every row and causes excessive server load. The simplest way to protect from this type of attack is to use single quotes around the numeric constants: SELECT * FROM table WHERE ID='234'. If the user enters extra information, it all becomes part of the string. In a numeric context, MySQL automatically converts this string to a number and strips any trailing non-numeric characters from it.
Sometimes people think that if a database contains only publicly available data, it need not be protected. This is incorrect. Even if it is allowable to display any row in the database, you should still protect against denial of service attacks (for example, those that are based on the technique in the preceding paragraph that causes the server to waste resources). Otherwise, your server becomes unresponsive to legitimate users.
Checklist:
- Try to enter single and double quote marks (“'” and “"”) in all of your Web forms. If you get any kind of MySQL error, investigate the problem right away.
- Try to modify dynamic URLs by adding %22 (“"”), %23 (“#”), and %27 (“'”) to them.
- Try to modify data types in dynamic URLs from numeric to character types using the characters shown in the previous examples. Your application should be safe against these and similar attacks.
- Try to enter characters, spaces, and special symbols rather than numbers in numeric fields. Your application should remove them before passing them to MySQL or else generate an error. Passing unchecked values to MySQL is very dangerous!
- Check the size of data before passing it to MySQL.
- Have your application connect to the database using a username different from the one you use for administrative purposes. Do not give your applications any access privileges they do not need.
Many application programming interfaces provide a means of escaping special characters in data values. Properly used, this prevents application users from entering values that cause the application to generate statements that have a different effect than you intend:
- MySQL C API: Use the mysql_real_escape_string() API call.
- MySQL++: Use the escape and quote modifiers for query streams.
- PHP: Use the mysql_real_escape_string() function (available as of PHP 4.3.0, prior to that PHP version use mysql_escape_string(), and prior to PHP 4.0.3, use addslashes() ). Note that only mysql_real_escape_string() is character set-aware; the other functions can be “bypassed” when using (invalid) multi-byte character sets. In PHP 5 (and as of MySQL 4.1), you can use the mysqli extension, which supports the improved MySQL authentication protocol and passwords, as well as prepared statements with placeholders.
- Perl DBI: Use placeholders or the quote() method.
- Ruby DBI: Use placeholders or the quote() method.
- Java JDBC: Use a PreparedStatement object and placeholders.
Other programming interfaces might have similar capabilities.
Do not transmit plain (unencrypted) data over the Internet. This information is accessible to everyone who has the time and ability to intercept it and use it for their own purposes. Instead, use an encrypted protocol such as SSL or SSH. MySQL supports internal SSL connections as of version 4.0. Another technique is to use SSH port-forwarding to create an encrypted (and compressed) tunnel for the communication.
Learn to use the tcpdump and strings utilities. In most cases, you can check whether MySQL data streams are unencrypted by issuing a command like the following:
```
shell> tcpdump -l -i eth0 -w - src or dst port 3306 | strings
```
This works under Linux and should work with small modifications under other systems.
Warning
If you do not see plaintext data, this does not always mean that the information actually is encrypted. If you need high security, you should consult with a security expert.

5.4.2. Making MySQL Secure Against Attackers

When you connect to a MySQL server, you should use a password. The password is not transmitted in clear text over the connection. Password handling during the client connection sequence was upgraded in MySQL 4.1.1 to be very secure. If you are still using pre-4.1.1-style passwords, the encryption algorithm is not as strong as the newer algorithm. With some effort, a clever attacker who can sniff the traffic between the client and the server can crack the password. (See Section 5.5.8, “Password Hashing as of MySQL 4.1”, for a discussion of the different password handling methods.)

MySQL Enterprise The MySQL Enterprise Monitor enforces best practices for maximizing the security of your servers. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

All other information is transferred as text, and can be read by anyone who is able to watch the connection. If the connection between the client and the server goes through an untrusted network, and you are concerned about this, you can use the compressed protocol (in MySQL 3.22 and above) to make traffic much more difficult to decipher. You can also use MySQL's internal SSL support to make the connection even more secure in MySQL 4.0 and up. See Section 5.6.7, “Using SSL for Secure Connections”. Alternatively, use SSH to get an encrypted TCP/IP connection between a MySQL server and a MySQL client. You can find an Open Source SSH client at http://www.openssh.org/, and a commercial SSH client at http://www.ssh.com/.

To make a MySQL system secure, you should strongly consider the following suggestions:

Require all MySQL accounts to have a password. A client program does not necessarily know the identity of the person running it. It is common for client/server applications that the user can specify any username to the client program. For example, anyone can use the mysql program to connect as any other person simply by invoking it as mysql -u other_user db_name if other_user has no password. If all accounts have a password, connecting using another user's account becomes much more difficult.
For a discussion of methods for setting passwords, see Section 5.6.5, “Assigning Account Passwords”.
Never run the MySQL server as the Unix root user. This is extremely dangerous, because any user with the FILE privilege is able to cause the server to create files as root (for example, ~root/.bashrc). To prevent this, mysqld refuses to run as root unless that is specified explicitly using the --user=root option.
mysqld can (and should) be run as an ordinary, unprivileged user instead. You can create a separate Unix account named mysql to make everything even more secure. Use this account only for administering MySQL. To start mysqld as a different Unix user, add a user option that specifies the username in the [mysqld] group of the my.cnf option file where you specify server options. For example:
```
[mysqld]
user=mysql
```
This causes the server to start as the designated user whether you start it manually or by using mysqld_safe or mysql.server. For more details, see Section 5.4.5, “How to Run MySQL as a Normal User”.
Running mysqld as a Unix user other than root does not mean that you need to change the root username in the user table. Usernames for MySQL accounts have nothing to do with usernames for Unix accounts.
Do not allow the use of symlinks to tables. (This capability can be disabled with the --skip-symbolic-links option.) This is especially important if you run mysqld as root, because anyone that has write access to the server's data directory then could delete any file in the system! See Section 7.6.1.2, “Using Symbolic Links for Tables on Unix”.
Make sure that the only Unix user account with read or write privileges in the database directories is the account that is used for running mysqld.
Do not grant the PROCESS or SUPER privilege to non-administrative users. The output of mysqladmin processlist and SHOW PROCESSLIST shows the text of any statements currently being executed, so any user who is allowed to see the server process list might be able to see statements issued by other users such as UPDATE user SET password=PASSWORD('not_secure').
mysqld reserves an extra connection for users who have the SUPER privilege (PROCESS before MySQL 4.0.2), so that a MySQL root user can log in and check server activity even if all normal connections are in use.
The SUPER privilege can be used to terminate client connections, change server operation by changing the value of system variables, and control replication servers.
Do not grant the FILE privilege to non-administrative users. Any user that has this privilege can write a file anywhere in the filesystem with the privileges of the mysqld daemon. To make this a bit safer, files generated with SELECT ... INTO OUTFILE do not overwrite existing files and are writable by everyone.
The FILE privilege may also be used to read any file that is world-readable or accessible to the Unix user that the server runs as. With this privilege, you can read any file into a database table. This could be abused, for example, by using LOAD DATA to load /etc/passwd into a table, which then can be displayed with SELECT.
If you do not trust your DNS, you should use IP numbers rather than hostnames in the grant tables. In any case, you should be very careful about creating grant table entries using hostname values that contain wildcards.
If you want to restrict the number of connections allowed to a single account, you can do so by setting the max_user_connections variable in mysqld. The GRANT statement also supports resource control options for limiting the extent of server use allowed to an account. See Section 12.5.1.2, “GRANT Syntax”.

5.4.3. Security-Related mysqld Options

The following mysqld options affect security:

Name	Cmd-Line	Option file	System Var	Var Scope	Dynamic
allow-suspicious-udfs	Yes	Yes
chroot	Yes	Yes
des-key-file	Yes	Yes
local_infile			Yes	Global	Yes
local-infile	Yes	Yes
old-passwords	Yes	Yes		Both	Yes
- Variable: old_passwords			Yes	Both	Yes
safe-show-database	Yes	Yes		Global	Yes
- Variable: safe_show_database			Yes	Global	Yes
safe-user-create	Yes	Yes
secure-auth	Yes	Yes		Global	Yes
- Variable: secure_auth			Yes	Global	Yes
skip-grant-tables	Yes	Yes
skip-name-resolve	Yes	Yes
skip-networking	Yes	Yes		Global	No
- Variable: skip_networking			Yes	Global	No
skip-show-database	Yes	Yes		Global	No
- Variable: skip_show_database			Yes	Global	No

--allow-suspicious-udfs
This option controls whether user-defined functions that have only an xxx symbol for the main function can be loaded. By default, the option is turned off and only UDFs that have at least one auxiliary symbol can be loaded; this prevents attempts at loading functions from shared object files other than those containing legitimate UDFs. This option was added in MySQL 4.0.24 and 4.1.10a. See Section 19.2.4.6, “User-Defined Function Security Precautions”.
--local-infile[={0|1}]
If you start the server with --local-infile=0, clients cannot use LOCAL in LOAD DATA statements. See Section 5.4.4, “Security Issues with LOAD DATA LOCAL”.
--old-passwords
Force the server to generate short (pre-4.1) password hashes for new passwords. This is useful for compatibility when the server must support older client programs. See Section 5.5.8, “Password Hashing as of MySQL 4.1”.
--safe-show-database
With this option, the SHOW DATABASES statement displays the names of only those databases for which the user has some kind of privilege. As of MySQL 4.0.2, this option is deprecated and does not do anything (it is enabled by default), because there is a SHOW DATABASES privilege that can be used to control access to database names on a per-account basis. See Section 12.5.1.2, “GRANT Syntax”.
--safe-user-create
If this option is enabled, a user cannot create new MySQL users by using the GRANT statement unless the user has the INSERT privilege for the mysql.user table. If you want a user to have the ability to create new users that have those privileges that the user has the right to grant, you should grant the user the following privilege:
```
GRANT INSERT(user) ON mysql.user TO 'user_name'@'host_name';
```
This ensures that the user cannot change any privilege columns directly, but has to use the GRANT statement to give privileges to other users.
--secure-auth
Disallow authentication for accounts that have old (pre-4.1) passwords. This option is available as of MySQL 4.1.1.
The mysql client also has a --secure-auth option, which prevents connections to a server if the server requires a password in old format for the client account.
--skip-grant-tables
This option causes the server not to use the privilege system at all. This gives anyone with access to the server unrestricted access to all databases. You can cause a running server to start using the grant tables again by executing mysqladmin flush-privileges or mysqladmin reload command from a system shell, or by issuing a MySQL FLUSH PRIVILEGES statement. This option also suppresses loading of user-defined functions (UDFs).
--skip-name-resolve
Hostnames are not resolved. All Host column values in the grant tables must be IP numbers or localhost.
--skip-networking
Do not allow TCP/IP connections over the network. All connections to mysqld must be made via Unix socket files. This option is unsuitable when using a MySQL version prior to 3.23.27 with the MIT-pthreads package, because Unix socket files were not supported by MIT-pthreads at that time.
--skip-show-database
With this option, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. Without this option, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database. Note that any global privilege is a privilege for the database.

5.4.4. Security Issues with `LOAD DATA LOCAL`

The LOAD DATA statement can load a file that is located on the server host, or it can load a file that is located on the client host when the LOCAL keyword is specified.

There are two potential security issues with supporting the LOCAL version of LOAD DATA statements:

The transfer of the file from the client host to the server host is initiated by the MySQL server. In theory, a patched server could be built that would tell the client program to transfer a file of the server's choosing rather than the file named by the client in the LOAD DATA statement. Such a server could access any file on the client host to which the client user has read access.
In a Web environment where the clients are connecting from a Web server, a user could use LOAD DATA LOCAL to read any files that the Web server process has read access to (assuming that a user could run any command against the SQL server). In this environment, the client with respect to the MySQL server actually is the Web server, not the remote program being run by the user who connects to the Web server.

To deal with these problems, we changed how LOAD DATA LOCAL is handled as of MySQL 3.23.49 and MySQL 4.0.2 (4.0.13 on Windows):

By default, all MySQL clients and libraries in binary distributions are compiled with the --enable-local-infile option, to be compatible with MySQL 3.23.48 and before.
If you build MySQL from source but do not invoke configure with the --enable-local-infile option, LOAD DATA LOCAL cannot be used by any client unless it is written explicitly to invoke mysql_options(... MYSQL_OPT_LOCAL_INFILE, 0). See Section 17.2.3.47, “mysql_options()”.
You can disable all LOAD DATA LOCAL commands from the server side by starting mysqld with the --local-infile=0 option.
For the mysql command-line client, LOAD DATA LOCAL can be enabled by specifying the --local-infile[=1] option, or disabled with the --local-infile=0 option. Similarly, for mysqlimport, the --local or -L option enables local data file loading. In any case, successful use of a local loading operation requires that the server is enabled to allow it.
If you use LOAD DATA LOCAL in Perl scripts or other programs that read the [client] group from option files, you can add the local-infile=1 option to that group. However, to keep this from causing problems for programs that do not understand local-infile, specify it using the loose- prefix:
```
[client]
loose-local-infile=1
```
The loose- prefix can be used as of MySQL 4.0.2.
If LOAD DATA LOCAL INFILE is disabled, either in the server or the client, a client that attempts to issue such a statement receives the following error message:
```
ERROR 1148: The used command is not allowed with this MySQL version
```
MySQL Enterprise Security advisors notify subscribers to the MySQL Enterprise Monitor whenever a server is started with the --local-infile option enabled. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

5.4.5. How to Run MySQL as a Normal User

On Windows, you can run the server as a Windows service using a normal user account beginning with MySQL 4.0.17 and 4.1.2. (Older MySQL versions required you to have administrator rights. This was a bug introduced in MySQL 3.23.54.)

On Unix, the MySQL server mysqld can be started and run by any user. However, you should avoid running the server as the Unix root user for security reasons. To change mysqld to run as a normal unprivileged Unix user user_name, you must do the following:

Stop the server if it's running (use mysqladmin shutdown).
Change the database directories and files so that user_name has privileges to read and write files in them (you might need to do this as the Unix root user):
```
shell> chown -R user_name /path/to/mysql/datadir
```
If you do not do this, the server will not be able to access databases or tables when it runs as user_name.
If directories or files within the MySQL data directory are symbolic links, you'll also need to follow those links and change the directories and files they point to. chown -R might not follow symbolic links for you.
Start the server as user user_name. If you are using MySQL 3.22 or later, another alternative is to start mysqld as the Unix root user and use the --user=user_name option. mysqld starts up, then switches to run as the Unix user user_name before accepting any connections.
To start the server as the given user automatically at system startup time, specify the username by adding a user option to the [mysqld] group of the /etc/my.cnf option file or the my.cnf option file in the server's data directory. For example:
```
[mysqld]
user=user_name
```

If your Unix machine itself isn't secured, you should assign passwords to the MySQL root accounts in the grant tables. Otherwise, any user with a login account on that machine can run the mysql client with a --user=root option and perform any operation. (It is a good idea to assign passwords to MySQL accounts in any case, but especially so when other login accounts exist on the server host.) See Section 2.10, “Post-Installation Setup and Testing”.

5.5. The MySQL Access Privilege System

5.5.1. What the Privilege System Does
5.5.2. How the Privilege System Works
5.5.3. Privileges Provided by MySQL
5.5.4. Access Control, Stage 1: Connection Verification
5.5.5. Access Control, Stage 2: Request Verification
5.5.6. When Privilege Changes Take Effect
5.5.7. Causes of Access denied Errors
5.5.8. Password Hashing as of MySQL 4.1

MySQL has an advanced but non-standard security and privilege system. The following discussion describes how it works.

5.5.1. What the Privilege System Does

The primary function of the MySQL privilege system is to authenticate a user who connects from a given host and to associate that user with privileges on a database such as SELECT, INSERT, UPDATE, and DELETE.

Additional functionality includes the ability to have anonymous users and to grant privileges for MySQL-specific functions such as LOAD DATA INFILE and administrative operations.

5.5.2. How the Privilege System Works

The MySQL privilege system ensures that all users may perform only the operations allowed to them. As a user, when you connect to a MySQL server, your identity is determined by the host from which you connect and the username you specify. When you issue requests after connecting, the system grants privileges according to your identity and what you want to do.

MySQL considers both your hostname and username in identifying you because there is little reason to assume that a given username belongs to the same person everywhere on the Internet. For example, the user joe who connects from office.example.com need not be the same person as the user joe who connects from home.example.com. MySQL handles this by allowing you to distinguish users on different hosts that happen to have the same name: You can grant one set of privileges for connections by joe from office.example.com, and a different set of privileges for connections by joe from home.example.com.

MySQL access control involves two stages when you run a client program that connects to the server:

Stage 1: The server checks whether it should allow you to connect.
Stage 2: Assuming that you can connect, the server checks each statement you issue to determine whether you have sufficient privileges to perform it. For example, if you try to select rows from a table in a database or drop a table from the database, the server verifies that you have the SELECT privilege for the table or the DROP privilege for the database.

If your privileges are changed (either by yourself or someone else) while you are connected, those changes do not necessarily take effect immediately for the next statement that you issue. See Section 5.5.6, “When Privilege Changes Take Effect”, for details.

The server stores privilege information in the grant tables of the mysql database (that is, in the database named mysql). The MySQL server reads the contents of these tables into memory when it starts and re-reads them under the circumstances indicated in Section 5.5.6, “When Privilege Changes Take Effect”. Access-control decisions are based on the in-memory copies of the grant tables.

Normally, you manipulate the contents of the grant tables indirectly by using statements such as GRANT and REVOKE to set up accounts and control the privileges available to each one. See Section 12.5.1, “Account Management Statements”. The discussion here describes the underlying structure of the grant tables and how the server uses their contents when interacting with clients.

The server uses the user, db, and host tables in the mysql database at both stages of access control. The columns in the user and db tables are shown here. The host table is similar to the db table but has a specialized use as described in Section 5.5.5, “Access Control, Stage 2: Request Verification”.

Table Name	user	db
Scope columns	`Host`	`Host`
	`User`	`Db`
	`Password`	`User`
Privilege columns	`Select_priv`	`Select_priv`
	`Insert_priv`	`Insert_priv`
	`Update_priv`	`Update_priv`
	`Delete_priv`	`Delete_priv`
	`Index_priv`	`Index_priv`
	`Alter_priv`	`Alter_priv`
	`Create_priv`	`Create_priv`
	`Drop_priv`	`Drop_priv`
	`Grant_priv`	`Grant_priv`
	`References_priv`	`References_priv`
	`Execute_priv`
	`Reload_priv`
	`Shutdown_priv`
	`Process_priv`
	`File_priv`
	`Show_db_priv`
	`Super_priv`
	`Create_tmp_table_priv`	`Create_tmp_table_priv`
	`Lock_tables_priv`	`Lock_tables_priv`
	`Repl_slave_priv`
	`Repl_client_priv`
Security columns	`ssl_type`
	`ssl_cipher`
	`x509_issuer`
	`x509_subject`
Resource control columns	`max_questions`
	`max_updates`
	`max_connections`
	`max_user_connections`

The ssl_type, ssl_cipher, x509_issuer, and x509_subject columns were added in MySQL 4.0.0.

The Create_tmp_table_priv, Execute_priv, Lock_tables_priv, Repl_client_priv, Repl_slave_priv, Show_db_priv, Super_priv, max_questions, max_updates, and max_connections columns were added in MySQL 4.0.2. Execute_priv is not operational through MySQL 4.1.

During the second stage of access control, the server performs request verification to make sure that each client has sufficient privileges for each request that it issues. In addition to the user, db, and host grant tables, the server may also consult the tables_priv and columns_priv tables for requests that involve tables. The tables_priv and columns_priv tables provide finer privilege control at the table and column levels. They have the following columns:

Table Name	tables_priv	columns_priv
Scope columns	`Host`	`Host`
	`Db`	`Db`
	`User`	`User`
	`Table_name`	`Table_name`
		`Column_name`
Privilege columns	`Table_priv`	`Column_priv`
	`Column_priv`
Other columns	`Timestamp`	`Timestamp`
	`Grantor`

The Timestamp and Grantor columns currently are unused and are discussed no further here.

Each grant table contains scope columns and privilege columns:

Scope columns determine the scope of each row (entry) in the tables; that is, the context in which the row applies. For example, a user table row with Host and User values of 'thomas.loc.gov' and 'bob' would be used for authenticating connections made to the server from the host thomas.loc.gov by a client that specifies a username of bob. Similarly, a db table row with Host, User, and Db column values of 'thomas.loc.gov', 'bob' and 'reports' would be used when bob connects from the host thomas.loc.gov to access the reports database. The tables_priv and columns_priv tables contain scope columns indicating tables or table/column combinations to which each row applies. The procs_priv scope columns indicate the stored routine to which each row applies.
Privilege columns indicate which privileges are granted by a table row; that is, what operations can be performed. The server combines the information in the various grant tables to form a complete description of a user's privileges. Section 5.5.5, “Access Control, Stage 2: Request Verification”, describes the rules that are used to do this.

Scope columns contain strings. They are declared as shown here; the default value for each is the empty string:

Column Name	Type
`Host`	`CHAR(60)`
`User`	`CHAR(16)`
`Password`	`CHAR(16)`
`Db`	`CHAR(64)`
`Table_name`	`CHAR(64)`
`Column_name`	`CHAR(64)`
`Routine_name`	`CHAR(64)`

Before MySQL 3.23, the Db column is CHAR(32) in some tables and CHAR(60) in others.

For access-checking purposes, comparisons of Host values are case insensitive. User, Password, Db, and Table_name values are case sensitive. Column_name values are case insensitive in MySQL 3.22.12 or later.

In the user, db, and host tables, each privilege is listed in a separate column that is declared as ENUM('N','Y') DEFAULT 'N'. In other words, each privilege can be disabled or enabled, with the default being disabled.

In the tables_priv, columns_priv, and procs_priv tables, the privilege columns are declared as SET columns. Values in these columns can contain any combination of the privileges controlled by the table:

Table Name	Column Name	Possible Set Elements
`tables_priv`	`Table_priv`	`'Select', 'Insert', 'Update', 'Delete', 'Create', 'Drop', 'Grant', 'References', 'Index', 'Alter'`
`tables_priv`	`Column_priv`	`'Select', 'Insert', 'Update', 'References'`
`columns_priv`	`Column_priv`	`'Select', 'Insert', 'Update', 'References'`

Briefly, the server uses the grant tables in the following manner:

The user table scope columns determine whether to reject or allow incoming connections. For allowed connections, any privileges granted in the user table indicate the user's global (superuser) privileges. Any privilege granted in this table applies to all databases on the server.
Note
Because any global privilege is considered a privilege for all databases, any global privilege enables a user to see all database names with SHOW DATABASES.
The db table scope columns determine which users can access which databases from which hosts. The privilege columns determine which operations are allowed. A privilege granted at the database level applies to the database and to all its tables.
The host table is used in conjunction with the db table when you want a given db table row to apply to several hosts. For example, if you want a user to be able to use a database from several hosts in your network, leave the Host value empty in the user's db table row, then populate the host table with a row for each of those hosts. This mechanism is described more detail in Section 5.5.5, “Access Control, Stage 2: Request Verification”.
Note
The host table must be modified directly with statements such as INSERT, UPDATE, and DELETE. It is not affected by statements such as GRANT and REVOKE that modify the grant tables indirectly. Most MySQL installations need not use this table at all.
The tables_priv and columns_priv tables are similar to the db table, but are more fine-grained: They apply at the table and column levels rather than at the database level. A privilege granted at the table level applies to the table and to all its columns. A privilege granted at the column level applies only to a specific column.

Administrative privileges (such as RELOAD or SHUTDOWN) are specified only in the user table. The reason for this is that administrative operations are operations on the server itself and are not database-specific, so there is no reason to list these privileges in the other grant tables. In fact, to determine whether you can perform an administrative operation, the server need consult only the user table.

The FILE privilege also is specified only in the user table. It is not an administrative privilege as such, but your ability to read or write files on the server host is independent of the database you are accessing.

The mysqld server reads the contents of the grant tables into memory when it starts. You can tell it to re-read the tables by issuing a FLUSH PRIVILEGES statement or executing a mysqladmin flush-privileges or mysqladmin reload command. Changes to the grant tables take effect as indicated in Section 5.5.6, “When Privilege Changes Take Effect”.

When you modify the contents of the grant tables, it is a good idea to make sure that your changes set up privileges the way you want. To check the privileges for a given account, use the SHOW GRANTS statement. (See Section 12.5.4.10, “SHOW GRANTS Syntax”.) For example, to determine the privileges that are granted to an account with Host and User values of pc84.example.com and bob, issue this statement:

SHOW GRANTS FOR 'bob'@'pc84.example.com';

For additional help in diagnosing privilege-related problems, see Section 5.5.7, “Causes of Access denied Errors”. For general advice on security issues, see Section 5.4, “General Security Issues”.

5.5.3. Privileges Provided by MySQL

Information about account privileges is stored in the user, db, host, tables_priv, and columns_priv tables in the mysql database. The MySQL server reads the contents of these tables into memory when it starts and re-reads them under the circumstances indicated in Section 5.5.6, “When Privilege Changes Take Effect”. Access-control decisions are based on the in-memory copies of the grant tables.

The names used in the GRANT and REVOKE statements to refer to privileges are shown in the following table, along with the column name associated with each privilege in the grant tables and the context in which the privilege applies. Further information about the meaning of each privilege may be found at Section 12.5.1.2, “GRANT Syntax”.

Privilege	Column	Context
`CREATE`	`Create_priv`	databases, tables, or indexes
`DROP`	`Drop_priv`	databases or tables
`GRANT OPTION`	`Grant_priv`	databases, tables, or stored routines
`REFERENCES`	`References_priv`	databases or tables (unused)
`ALTER`	`Alter_priv`	tables
`DELETE`	`Delete_priv`	tables
`INDEX`	`Index_priv`	tables
`INSERT`	`Insert_priv`	tables
`SELECT`	`Select_priv`	tables
`UPDATE`	`Update_priv`	tables
`FILE`	`File_priv`	file access on server host
`CREATE TEMPORARY TABLES`	`Create_tmp_table_priv`	server administration
`LOCK TABLES`	`Lock_tables_priv`	server administration
`PROCESS`	`Process_priv`	server administration
`RELOAD`	`Reload_priv`	server administration
`REPLICATION CLIENT`	`Repl_client_priv`	server administration
`REPLICATION SLAVE`	`Repl_slave_priv`	server administration
`SHOW DATABASES`	`Show_db_priv`	server administration
`SHUTDOWN`	`Shutdown_priv`	server administration
`SUPER`	`Super_priv`	server administration

Some releases of MySQL introduce changes to the structure of the grant tables to add new privileges or features. Whenever you update to a new version of MySQL, you should update your grant tables to make sure that they have the current structure so that you can take advantage of any new capabilities. See Section 4.4.5, “mysql_fix_privilege_tables — Upgrade MySQL System Tables”.

The CREATE TEMPORARY TABLES, EXECUTE, LOCK TABLES, REPLICATION CLIENT, REPLICATION SLAVE, SHOW DATABASES, and SUPER privileges were added in MySQL 4.0.2. (EXECUTE is not used in any MySQL version through the 4.1 release series.)

The CREATE and DROP privileges allow you to create new databases and tables, or to drop (remove) existing databases and tables. If you grant the DROP privilege for the mysql database to a user, that user can drop the database in which the MySQL access privileges are stored!

The SELECT, INSERT, UPDATE, and DELETE privileges allow you to perform operations on rows in existing tables in a database. INSERT is also required for the ANALYZE TABLE, OPTIMIZE TABLE, and REPAIR TABLE table-maintenance statements.

SELECT statements require the SELECT privilege only if they actually retrieve rows from a table. Some SELECT statements do not access tables and can be executed without permission for any database. For example, you can use the mysql client as a simple calculator to evaluate expressions that make no reference to tables:

SELECT 1+1;
SELECT PI()*2;

The INDEX privilege enables you to create or drop (remove) indexes. INDEX applies to existing tables. If you have the CREATE privilege for a table, you can include index definitions in the CREATE TABLE statement.

The ALTER privilege enables you to use ALTER TABLE to change the structure of or rename tables.

The GRANT privilege enables you to give to other users those privileges that you yourself possess. It can be used for databases, tables, and stored routines.

The FILE privilege gives you permission to read and write files on the server host using the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements. A user who has the FILE privilege can read any file on the server host that is either world-readable or readable by the MySQL server. (This implies the user can read any file in any database directory, because the server can access any of those files.) The FILE privilege also enables the user to create new files in any directory where the MySQL server has write access. As a security measure, the server will not overwrite existing files.

The REFERENCES privilege currently is unused.

The remaining privileges are used for administrative operations. Many of them can be performed by using the mysqladmin program or by issuing SQL statements. The following table shows which mysqladmin commands each administrative privilege enables you to execute:

Privilege	Commands Permitted to Privilege Holders
`RELOAD`	`flush-hosts`, `flush-logs`, `flush-privileges`, `flush-status`, `flush-tables`, `flush-threads`, `refresh`, `reload`
`SHUTDOWN`	`shutdown`
`PROCESS`	`processlist`
`SUPER`	`kill`

The reload command tells the server to re-read the grant tables into memory. flush-privileges is a synonym for reload. The refresh command closes and reopens the log files and flushes all tables. The other flush-xxx commands perform functions similar to refresh, but are more specific and may be preferable in some instances. For example, if you want to flush just the log files, flush-logs is a better choice than refresh.

The shutdown command shuts down the server. There is no corresponding SQL statement.

The processlist command displays information about the threads executing within the server (that is, information about the statements being executed by clients). The kill command terminates server threads. You can always display or kill your own threads, but you need the PROCESS privilege to display threads initiated by other users and the SUPER privilege to kill them. See Section 12.5.5.3, “KILL Syntax”. Prior to MySQL 4.0.2 when SUPER was introduced, the PROCESS privilege controls the ability to both see and terminate threads for other clients.

The CREATE TEMPORARY TABLES privilege enables the use of the keyword TEMPORARY in CREATE TABLE statements.

The LOCK TABLES privilege enables the use of explicit LOCK TABLES statements to lock tables for which you have the SELECT privilege. This includes the use of write locks, which prevents anyone else from reading the locked table.

The REPLICATION CLIENT privilege enables the use of SHOW MASTER STATUS and SHOW SLAVE STATUS.

The REPLICATION SLAVE privilege should be granted to accounts that are used by slave servers to connect to the current server as their master. Without this privilege, the slave cannot request updates that have been made to databases on the master server.

The SHOW DATABASES privilege allows the account to see database names by issuing the SHOW DATABASE statement. Accounts that do not have this privilege see only databases for which they have some privileges, and cannot use the statement at all if the server was started with the --skip-show-database option. Note that any global privilege is a privilege for the database.

It is a good idea to grant to an account only those privileges that it needs. You should exercise particular caution in granting the FILE and administrative privileges:

The FILE privilege can be abused to read into a database table any files that the MySQL server can read on the server host. This includes all world-readable files and files in the server's data directory. The table can then be accessed using SELECT to transfer its contents to the client host.
The GRANT privilege enables users to give their privileges to other users. Two users that have different privileges and with the GRANT privilege are able to combine privileges.
The ALTER privilege may be used to subvert the privilege system by renaming tables.
The SHUTDOWN privilege can be abused to deny service to other users entirely by terminating the server.
The PROCESS privilege can be used to view the plain text of currently executing statements, including statements that set or change passwords.
The SUPER privilege can be used to terminate other clients or change how the server operates.
Privileges granted for the mysql database itself can be used to change passwords and other access privilege information. Passwords are stored encrypted, so a malicious user cannot simply read them to know the plain text password. However, a user with write access to the user table Password column can change an account's password, and then connect to the MySQL server using that account.
MySQL Enterprise Accounts with unnecessary global privileges constitute a security risk. Subscribers to the MySQL Enterprise Monitor are automatically alerted to the existence of such accounts. For detailed information, see http://www.mysql.com/products/enterprise/advisors.html.

There are some things that you cannot do with the MySQL privilege system:

You cannot explicitly specify that a given user should be denied access. That is, you cannot explicitly match a user and then refuse the connection.
You cannot specify that a user has privileges to create or drop tables in a database but not to create or drop the database itself.
A password applies globally to an account. You cannot associate a password with a specific object such as a database or table.

5.5.4. Access Control, Stage 1: Connection Verification

When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether you can verify your identity by supplying the correct password. If not, the server denies access to you completely. Otherwise, the server accepts the connection, and then enters Stage 2 and waits for requests.

Your identity is based on two pieces of information:

The client host from which you connect
Your MySQL username

Identity checking is performed using the three user table scope columns (Host, User, and Password). The server accepts the connection only if the Host and User columns in some user table row match the client hostname and username and the client supplies the password specified in that row.

Host values in the user table may be specified as follows:

A Host value may be a hostname or an IP number, or 'localhost' to indicate the local host.
You can use the wildcard characters “%” and “_” in Host column values. These have the same meaning as for pattern-matching operations performed with the LIKE operator. For example, a Host value of '%' matches any hostname, whereas a value of '%.mysql.com' matches any host in the mysql.com domain.
MySQL Enterprise An overly broad host specifier such as “%” constitutes a security risk. The MySQL Enterprise Monitor provides safeguards against this kind of vulnerability. For more information, see http://www.mysql.com/products/enterprise/advisors.html.
As of MySQL 3.23, for Host values specified as IP numbers, you can specify a netmask indicating how many address bits to use for the network number. For example:
```
GRANT ALL PRIVILEGES ON db.* TO david@'192.58.197.0/255.255.255.0';
```
This allows david to connect from any client host having an IP number client_ip for which the following condition is true:
```
client_ip & netmask = host_ip
```
That is, for the GRANT statement just shown:
```
client_ip & 255.255.255.0 = 192.58.197.0
```
IP numbers that satisfy this condition and can connect to the MySQL server are those in the range from 192.58.197.0 to 192.58.197.255.
Note: The netmask can only be used to tell the server to use 8, 16, 24, or 32 bits of the address. Examples:
- 192.0.0.0/255.0.0.0: anything on the 192 class A network
- 192.168.0.0/255.255.0.0: anything on the 192.168 class B network
- 192.168.1.0/255.255.255.0: anything on the 192.168.1 class C network
- 192.168.1.1: only this specific IP
The following netmask (28 bits) will not work:
```
192.168.0.1/255.255.255.240
```
A blank Host value in a db table row means that its privileges should be combined with those in the row in the host table that matches the client hostname. The privileges are combined using an AND (intersection) operation, not OR (union). Section 5.5.5, “Access Control, Stage 2: Request Verification”, discusses use of the host table further.
A blank Host value in the other grant tables is the same as '%'.

Because you can use IP wildcard values in the Host column (for example, '144.155.166.%' to match every host on a subnet), someone could try to exploit this capability by naming a host 144.155.166.somewhere.com. To foil such attempts, MySQL disallows matching on hostnames that start with digits and a dot. Thus, if you have a host named something like 1.2.foo.com, its name never matches the Host column of the grant tables. An IP wildcard value can match only IP numbers, not hostnames.

In the User column, wildcard characters are not allowed, but you can specify a blank value, which matches any name. If the user table row that matches an incoming connection has a blank username, the user is considered to be an anonymous user with no name, not a user with the name that the client actually specified. This means that a blank username is used for all further access checking for the duration of the connection (that is, during Stage 2).

The Password column can be blank. This is not a wildcard and does not mean that any password matches. It means that the user must connect without specifying a password.

Non-blank Password values in the user table represent encrypted passwords. MySQL does not store passwords in plaintext form for anyone to see. Rather, the password supplied by a user who is attempting to connect is encrypted (using the PASSWORD() function). The encrypted password then is used during the connection process when checking whether the password is correct. (This is done without the encrypted password ever traveling over the connection.) From MySQL's point of view, the encrypted password is the real password, so you should never give anyone access to it. In particular, do not give non-administrative users read access to tables in the mysql database.

From version 4.1 on, MySQL employs a stronger authentication method that has better password protection during the connection process than in earlier versions. It is secure even if TCP/IP packets are sniffed or the mysql database is captured. Section 5.5.8, “Password Hashing as of MySQL 4.1”, discusses password encryption further.

The following table shows how various combinations of Host and User values in the user table apply to incoming connections.

`Host` Value	`User` Value	Allowable Connections
`'thomas.loc.gov'`	`'fred'`	`fred`, connecting from `thomas.loc.gov`
`'thomas.loc.gov'`	`''`	Any user, connecting from `thomas.loc.gov`
`'%'`	`'fred'`	`fred`, connecting from any host
`'%'`	`''`	Any user, connecting from any host
`'%.loc.gov'`	`'fred'`	`fred`, connecting from any host in the `loc.gov` domain
`'x.y.%'`	`'fred'`	`fred`, connecting from `x.y.net`, `x.y.com`, `x.y.edu`, and so on (this is probably not useful)
`'144.155.166.177'`	`'fred'`	`fred`, connecting from the host with IP address `144.155.166.177`
`'144.155.166.%'`	`'fred'`	`fred`, connecting from any host in the `144.155.166` class C subnet
`'144.155.166.0/255.255.255.0'`	`'fred'`	Same as previous example

It is possible for the client hostname and username of an incoming connection to match more than one row in the user table. The preceding set of examples demonstrates this: Several of the entries shown match a connection from thomas.loc.gov by fred.

When multiple matches are possible, the server must determine which of them to use. It resolves this issue as follows:

Whenever the server reads the user table into memory, it sorts the rows.
When a client attempts to connect, the server looks through the rows in sorted order.
The server uses the first row that matches the client hostname and username.

To see how this works, suppose that the user table looks like this:

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| %         | root     | ...
| %         | jeffrey  | ...
| localhost | root     | ...
| localhost |          | ...
+-----------+----------+-

When the server reads the table into memory, it orders the rows with the most-specific Host values first. Literal hostnames and IP numbers are the most specific. The pattern '%' means “any host” and is least specific. Rows with the same Host value are ordered with the most-specific User values first (a blank User value means “any user” and is least specific). For the user table just shown, the result after sorting looks like this:

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| localhost | root     | ...
| localhost |          | ...
| %         | jeffrey  | ...
| %         | root     | ...
+-----------+----------+-

When a client attempts to connect, the server looks through the sorted rows and uses the first match found. For a connection from localhost by jeffrey, two of the rows from the table match: the one with Host and User values of 'localhost' and '', and the one with values of '%' and 'jeffrey'. The 'localhost' row appears first in sorted order, so that is the one the server uses.

Here is another example. Suppose that the user table looks like this:

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| %              | jeffrey  | ...
| thomas.loc.gov |          | ...
+----------------+----------+-

The sorted table looks like this:

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| thomas.loc.gov |          | ...
| %              | jeffrey  | ...
+----------------+----------+-

A connection by jeffrey from thomas.loc.gov is matched by the first row, whereas a connection by jeffrey from whitehouse.gov is matched by the second.

It is a common misconception to think that, for a given username, all rows that explicitly name that user are used first when the server attempts to find a match for the connection. This is simply not true. The previous example illustrates this, where a connection from thomas.loc.gov by jeffrey is first matched not by the row containing 'jeffrey' as the User column value, but by the row with no username. As a result, jeffrey is authenticated as an anonymous user, even though he specified a username when connecting.

If you are able to connect to the server, but your privileges are not what you expect, you probably are being authenticated as some other account. To find out what account the server used to authenticate you, use the CURRENT_USER() function. (See Section 11.10.3, “Information Functions”.) It returns a value in user_name@host_name format that indicates the User and Host values from the matching user table row. Suppose that jeffrey connects and issues the following query:

mysql> SELECT CURRENT_USER();
+----------------+
| CURRENT_USER() |
+----------------+
| @localhost     |
+----------------+

The result shown here indicates that the matching user table row had a blank User column value. In other words, the server is treating jeffrey as an anonymous user.

The CURRENT_USER() function is available as of MySQL 4.0.6. See Section 11.10.3, “Information Functions”. Another way to diagnose authentication problems is to print out the user table and sort it by hand to see where the first match is being made.

5.5.5. Access Control, Stage 2: Request Verification

After you establish a connection, the server enters Stage 2 of access control. For each request that you issue via that connection, the server determines what operation you want to perform, then checks whether you have sufficient privileges to do so. This is where the privilege columns in the grant tables come into play. These privileges can come from any of the user, db, host, tables_priv, or columns_priv tables. (You may find it helpful to refer to Section 5.5.2, “How the Privilege System Works”, which lists the columns present in each of the grant tables.)

The user table grants privileges that are assigned to you on a global basis and that apply no matter what the default database is. For example, if the user table grants you the DELETE privilege, you can delete rows from any table in any database on the server host! In other words, user table privileges are superuser privileges. It is wise to grant privileges in the user table only to superusers such as database administrators. For other users, you should leave all privileges in the user table set to 'N' and grant privileges at more specific levels only. You can grant privileges for particular databases, tables, or columns.

The db and host tables grant database-specific privileges. Values in the scope columns of these tables can take the following forms:

The wildcard characters “%” and “_” can be used in the Host and Db columns of either table. These have the same meaning as for pattern-matching operations performed with the LIKE operator. If you want to use either character literally when granting privileges, you must escape it with a backslash. For example, to include “_” character as part of a database name, specify it as “\_” in the GRANT statement.
A '%' Host value in the db table means “any host.” A blank Host value in the db table means “consult the host table for further information” (a process that is described later in this section).
A '%' or blank Host value in the host table means “any host.”
A '%' or blank Db value in either table means “any database.”
A blank User value in the db table matches the anonymous user.

The server reads the db and host tables into memory and sorts them at the same time that it reads the user table. The server sorts the db table based on the Host, Db, and User scope columns, and sorts the host table based on the Host and Db scope columns. As with the user table, sorting puts the most-specific values first and least-specific values last, and when the server looks for matching entries, it uses the first match that it finds.

The tables_priv and columns_priv tables grant table-specific and column-specific privileges. Values in the scope columns of these tables can take the following forms:

The wildcard characters “%” and “_” can be used in the Host column. These have the same meaning as for pattern-matching operations performed with the LIKE operator.
A '%' or blank Host value means “any host.”
The Db, Table_name, and Column_name columns cannot contain wildcards or be blank.

The server sorts the tables_priv and columns_priv tables based on the Host, Db, and User columns. This is similar to db table sorting, but simpler because only the Host column can contain wildcards.

The server uses the sorted tables to verify each request that it receives. For requests that require administrative privileges such as SHUTDOWN or RELOAD, the server checks only the user table row because that is the only table that specifies administrative privileges. The server grants access if the row allows the requested operation and denies access otherwise. For example, if you want to execute mysqladmin shutdown but your user table row does not grant the SHUTDOWN privilege to you, the server denies access without even checking the db or host tables. (They contain no Shutdown_priv column, so there is no need to do so.)

For database-related requests (INSERT, UPDATE, and so on), the server first checks the user's global (superuser) privileges by looking in the user table row. If the row allows the requested operation, access is granted. If the global privileges in the user table are insufficient, the server determines the user's database-specific privileges by checking the db and host tables:

The server looks in the db table for a match on the Host, Db, and User columns. The Host and User columns are matched to the connecting user's hostname and MySQL username. The Db column is matched to the database that the user wants to access. If there is no row for the Host and User, access is denied.
If there is a matching db table row and its Host column is not blank, that row defines the user's database-specific privileges.
If the matching db table row's Host column is blank, it signifies that the host table enumerates which hosts should be allowed access to the database. In this case, a further lookup is done in the host table to find a match on the Host and Db columns. If no host table row matches, access is denied. If there is a match, the user's database-specific privileges are computed as the intersection (not the union!) of the privileges in the db and host table entries; that is, the privileges that are 'Y' in both entries. (This way you can grant general privileges in the db table row and then selectively restrict them on a host-by-host basis using the host table entries.)

After determining the database-specific privileges granted by the db and host table entries, the server adds them to the global privileges granted by the user table. If the result allows the requested operation, access is granted. Otherwise, the server successively checks the user's table and column privileges in the tables_priv and columns_priv tables, adds those to the user's privileges, and allows or denies access based on the result.

Expressed in boolean terms, the preceding description of how a user's privileges are calculated may be summarized like this:

global privileges
OR (database privileges AND host privileges)
OR table privileges
OR column privileges

It may not be apparent why, if the global user row privileges are initially found to be insufficient for the requested operation, the server adds those privileges to the database, table, and column privileges later. The reason is that a request might require more than one type of privilege. For example, if you execute an INSERT INTO ... SELECT statement, you need both the INSERT and the SELECT privileges. Your privileges might be such that the user table row grants one privilege and the db table row grants the other. In this case, you have the necessary privileges to perform the request, but the server cannot tell that from either table by itself; the privileges granted by the entries in both tables must be combined.

The host table is not affected by the GRANT or REVOKE statements, so it is unused in most MySQL installations. If you modify it directly, you can use it for some specialized purposes, such as to maintain a list of secure servers. For example, at TcX, the host table contains a list of all machines on the local network. These are granted all privileges.

You can also use the host table to indicate hosts that are not secure. Suppose that you have a machine public.your.domain that is located in a public area that you do not consider secure. You can allow access to all hosts on your network except that machine by using host table entries like this:

+--------------------+----+-
| Host               | Db | ...
+--------------------+----+-
| public.your.domain | %  | ... (all privileges set to 'N')
| %.your.domain      | %  | ... (all privileges set to 'Y')
+--------------------+----+-

Naturally, you should always test your changes to the grant tables (for example, by using SHOW GRANTS) to make sure that your access privileges are actually set up the way you think they are.

5.5.6. When Privilege Changes Take Effect

When mysqld starts, it reads all grant table contents into memory. The in-memory tables become effective for access control at that point.

When the server reloads the grant tables, privileges for existing client connections are affected as follows:

Table and column privilege changes take effect with the client's next request.
Database privilege changes take effect at the next USE db_name statement.
Note
Client applications may cache the database name; thus, this effect may not be visible to them without actually changing to a different database or executing a FLUSH PRIVILEGES statement.
Changes to global privileges and passwords take effect the next time the client connects.

If you modify the grant tables indirectly using statements such as GRANT, REVOKE, or SET PASSWORD, the server notices these changes and loads the grant tables into memory again immediately.

If you modify the grant tables directly using statements such as INSERT, UPDATE, or DELETE, your changes have no effect on privilege checking until you either restart the server or tell it to reload the tables. To reload the grant tables manually, issue a FLUSH PRIVILEGES statement or execute a mysqladmin flush-privileges or mysqladmin reload command.

If you change the grant tables directly but forget to reload them, your changes have no effect until you restart the server. This may leave you wondering why your changes do not seem to make any difference!

5.5.7. Causes of `Access denied` Errors

If you encounter problems when you try to connect to the MySQL server, the following items describe some courses of action you can take to correct the problem.

Make sure that the server is running. If it is not running, you cannot connect to it. For example, if you attempt to connect to the server and see a message such as one of those following, one cause might be that the server is not running:
```
shell> mysql
ERROR 2003: Can't connect to MySQL server on 'host_name' (111)
shell> mysql
ERROR 2002: Can't connect to local MySQL server through socket
'/tmp/mysql.sock' (111)
```
It might also be that the server is running, but you are trying to connect using a TCP/IP port, named pipe, or Unix socket file different from the one on which the server is listening. To correct this when you invoke a client program, specify a --port option to indicate the proper port number, or a --socket option to indicate the proper named pipe or Unix socket file. To find out where the socket file is, you can use this command:
```
shell> netstat -ln | grep mysql
```
The grant tables must be properly set up so that the server can use them for access control. For some distribution types (such as binary distributions on Windows, or RPM distributions on Linux), the installation process initializes the mysql database containing the grant tables. For distributions that do not do this, you must initialize the grant tables manually by running the mysql_install_db script. For details, see Section 2.10.2, “Unix Post-Installation Procedures”.
One way to determine whether you need to initialize the grant tables is to look for a mysql directory under the data directory. (The data directory normally is named data or var and is located under your MySQL installation directory.) Make sure that you have a file named user.MYD in the mysql database directory. If you do not, execute the mysql_install_db script. After running this script and starting the server, test the initial privileges by executing this command:
```
shell> mysql -u root test
```
The server should let you connect without error.
After a fresh installation, you should connect to the server and set up your users and their access permissions:
```
shell> mysql -u root mysql
```
The server should let you connect because the MySQL root user has no password initially. That is also a security risk, so setting the password for the root accounts is something you should do while you're setting up your other MySQL accounts. For instructions on setting the initial passwords, see Section 2.10.3, “Securing the Initial MySQL Accounts”.
MySQL Enterprise The MySQL Enterprise Monitor enforces security-related best practices. For example, subscribers are alerted whenever there is any account without a password. For more information, see http://www.mysql.com/products/enterprise/advisors.html.
If you have updated an existing MySQL installation to a newer version, did you run the mysql_fix_privilege_tables script? If not, do so. The structure of the grant tables changes occasionally when new capabilities are added, so after an upgrade you should always make sure that your tables have the current structure. For instructions, see Section 4.4.5, “mysql_fix_privilege_tables — Upgrade MySQL System Tables”.
If a client program receives the following error message when it tries to connect, it means that the server expects passwords in a newer format than the client is capable of generating:
```
shell> mysql
Client does not support authentication protocol requested
by server; consider upgrading MySQL client
```
For information on how to deal with this, see Section 5.5.8, “Password Hashing as of MySQL 4.1”, and Section A.1.2.4, “Client does not support authentication protocol”.
If you try to connect as root and get the following error, it means that you do not have an row in the user table with a User column value of 'root' and that mysqld cannot resolve the hostname for your client:
```
Access denied for user ''@'unknown' to database mysql
```
In this case, you must restart the server with the --skip-grant-tables option and edit your /etc/hosts file or \windows\hosts file to add an entry for your host.
Remember that client programs use connection parameters specified in option files or environment variables. If a client program seems to be sending incorrect default connection parameters when you have not specified them on the command line, check your environment and any applicable option files. For example, if you get Access denied when you run a client without any options, make sure that you have not specified an old password in any of your option files!
You can suppress the use of option files by a client program by invoking it with the --no-defaults option. For example:
```
shell> mysqladmin --no-defaults -u root version
```
The option files that clients use are listed in Section 4.2.3.2, “Using Option Files”. Environment variables are listed in Section 2.14, “Environment Variables”.
If you get the following error, it means that you are using an incorrect root password:
```
shell> mysqladmin -u root -pxxxx ver
Access denied for user 'root'@'localhost' (using password: YES)
```
If the preceding error occurs even when you have not specified a password, it means that you have an incorrect password listed in some option file. Try the --no-defaults option as described in the previous item.
For information on changing passwords, see Section 5.6.5, “Assigning Account Passwords”.
If you have lost or forgotten the root password, you can restart mysqld with --skip-grant-tables to change the password. See Section A.1.4.1, “How to Reset the Root Password”.
If you change a password by using SET PASSWORD, INSERT, or UPDATE, you must encrypt the password using the PASSWORD() function. If you do not use PASSWORD() for these statements, the password will not work. For example, the following statement sets a password, but fails to encrypt it, so the user is not able to connect afterward:
```
SET PASSWORD FOR 'abe'@'host_name' = 'eagle';
```
Instead, set the password like this:
```
SET PASSWORD FOR 'abe'@'host_name' = PASSWORD('eagle');
```
The PASSWORD() function is unnecessary when you specify a password using the GRANT statement or the mysqladmin password command. Each of those automatically uses PASSWORD() to encrypt the password. See Section 5.6.5, “Assigning Account Passwords”.
localhost is a synonym for your local hostname, and is also the default host to which clients try to connect if you specify no host explicitly. However, connections to localhost on Unix systems do not work if you are using a MySQL version older than 3.23.27 that uses MIT-pthreads: localhost connections are made using Unix socket files, which were not supported by MIT-pthreads at that time.
To avoid this problem on such systems, you can use a --host=127.0.0.1 option to name the server host explicitly. This will make a TCP/IP connection to the local mysqld server. You can also use TCP/IP by specifying a --host option that uses the actual hostname of the local host. In this case, the hostname must be specified in a user table row on the server host, even though you are running the client program on the same host as the server.
If you get an Access denied error when trying to connect to the database with mysql -u user_name, you may have a problem with the user table. Check this by executing mysql -u root mysql and issuing this SQL statement:
```
SELECT * FROM user;
```
The result should include a row with the Host and User columns matching your computer's hostname and your MySQL username.
The Access denied error message tells you who you are trying to log in as, the client host from which you are trying to connect, and whether you were using a password. Normally, you should have one row in the user table that exactly matches the hostname and username that were given in the error message. For example, if you get an error message that contains using password: NO, it means that you tried to log in without a password.
If the following error occurs when you try to connect from a host other than the one on which the MySQL server is running, it means that there is no row in the user table with a Host value that matches the client host:
```
Host ... is not allowed to connect to this MySQL server
```
You can fix this by setting up an account for the combination of client hostname and username that you are using when trying to connect.
If you do not know the IP number or hostname of the machine from which you are connecting, you should put a row with '%' as the Host column value in the user table. After trying to connect from the client machine, use a SELECT USER() query to see how you really did connect. (Then change the '%' in the user table row to the actual hostname that shows up in the log. Otherwise, your system is left insecure because it allows connections from any host for the given username.)
(Note that if you are running a version of MySQL older than 3.23.11, the output from USER() does not include the hostname. In this case, you must restart the server with the --log option, then obtain the hostname from the log.)
On Linux, another reason that this error might occur is that you are using a binary MySQL version that is compiled with a different version of the glibc library than the one you are using. In this case, you should either upgrade your operating system or glibc, or download a source distribution of MySQL version and compile it yourself. A source RPM is normally trivial to compile and install, so this is not a big problem.
If you specify a hostname when trying to connect, but get an error message where the hostname is not shown or is an IP number, it means that the MySQL server got an error when trying to resolve the IP number of the client host to a name:
```
shell> mysqladmin -u root -pxxxx -h some_hostname ver
Access denied for user 'root'@'' (using password: YES)
```
This indicates a DNS problem. To fix it, execute mysqladmin flush-hosts to reset the internal DNS hostname cache. See Section 7.5.9, “How MySQL Uses DNS”.
Some permanent solutions are:
- Determine what is wrong with your DNS server and fix it.
- Specify IP numbers rather than hostnames in the MySQL grant tables.
- Put an entry for the client machine name in /etc/hosts or \windows\hosts.
- Start mysqld with the --skip-name-resolve option.
- Start mysqld with the --skip-host-cache option.
- On Unix, if you are running the server and the client on the same machine, connect to localhost. Unix connections to localhost use a Unix socket file rather than TCP/IP.
- On Windows, if you are running the server and the client on the same machine and the server supports named pipe connections, connect to the hostname . (period). Connections to . use a named pipe rather than TCP/IP.
If mysql -u root test works but mysql -h your_hostname -u root test results in Access denied (where your_hostname is the actual hostname of the local host), you may not have the correct name for your host in the user table. A common problem here is that the Host value in the user table row specifies an unqualified hostname, but your system's name resolution routines return a fully qualified domain name (or vice versa). For example, if you have an entry with host 'tcx' in the user table, but your DNS tells MySQL that your hostname is 'tcx.subnet.se', the entry does not work. Try adding an entry to the user table that contains the IP number of your host as the Host column value. (Alternatively, you could add an entry to the user table with a Host value that contains a wildcard; for example, 'tcx.%'. However, use of hostnames ending with “%” is insecure and is not recommended.)
If mysql -u user_name test works but mysql -u user_name other_db_name does not, you have not granted database access for other_db_name to the given user.
If mysql -u user_name works when executed on the server host, but mysql -h host_name -u user_name does not work when executed on a remote client host, you have not enabled access to the server for the given username from the remote host.
If you cannot figure out why you get Access denied, remove from the user table all entries that have Host values containing wildcards (entries that contain “%” or “_”). A very common error is to insert a new entry with Host='%' and User='some_user', thinking that this allows you to specify localhost to connect from the same machine. The reason that this does not work is that the default privileges include an entry with Host='localhost' and User=''. Because that entry has a Host value 'localhost' that is more specific than '%', it is used in preference to the new entry when connecting from localhost! The correct procedure is to insert a second entry with Host='localhost' and User='some_user', or to delete the entry with Host='localhost' and User=''. After deleting the entry, remember to issue a FLUSH PRIVILEGES statement to reload the grant tables.
If you get the following error, you may have a problem with the db or host table:
```
Access to database denied
```
If the entry selected from the db table has an empty value in the Host column, make sure that there are one or more corresponding entries in the host table specifying which hosts the db table entry applies to.
If you are able to connect to the MySQL server, but get an Access denied message whenever you issue a SELECT ... INTO OUTFILE or LOAD DATA INFILE statement, your entry in the user table does not have the FILE privilege enabled.
If you change the grant tables directly (for example, by using INSERT, UPDATE, or DELETE statements) and your changes seem to be ignored, remember that you must execute a FLUSH PRIVILEGES statement or a mysqladmin flush-privileges command to cause the server to re-read the privilege tables. Otherwise, your changes have no effect until the next time the server is restarted. Remember that after you change the root password with an UPDATE command, you will not need to specify the new password until after you flush the privileges, because the server will not know you've changed the password yet!
If your privileges seem to have changed in the middle of a session, it may be that a MySQL administrator has changed them. Reloading the grant tables affects new client connections, but it also affects existing connections as indicated in Section 5.5.6, “When Privilege Changes Take Effect”.
If you have access problems with a Perl, PHP, Python, or ODBC program, try to connect to the server with mysql -u user_name db_name or mysql -u user_name -pyour_pass db_name. If you are able to connect using the mysql client, the problem lies with your program, not with the access privileges. (There is no space between -p and the password; you can also use the --password=your_pass syntax to specify the password. If you use the -p --passwordoption with no password value, MySQL prompts you for the password.)
For testing, start the mysqld server with the --skip-grant-tables option. Then you can change the MySQL grant tables and use the mysqlaccess script to check whether your modifications have the desired effect. When you are satisfied with your changes, execute mysqladmin flush-privileges to tell the mysqld server to start using the new grant tables. (Reloading the grant tables overrides the --skip-grant-tables option. This enables you to tell the server to begin using the grant tables again without stopping and restarting it.)
If everything else fails, start the mysqld server with a debugging option (for example, --debug=d,general,query). This prints host and user information about attempted connections, as well as information about each command issued. See MySQL Internals: Porting.
If you have any other problems with the MySQL grant tables and feel you must post the problem to the mailing list, always provide a dump of the MySQL grant tables. You can dump the tables with the mysqldump mysql command. To file a bug report, see the instructions at Section 1.7, “How to Report Bugs or Problems”. In some cases, you may need to restart mysqld with --skip-grant-tables to run mysqldump.

5.5.8. Password Hashing as of MySQL 4.1

5.5.8.1. Implications of Password Hashing Changes for Application Programs
5.5.8.2. Password Hashing in MySQL 4.1.0

MySQL user accounts are listed in the user table of the mysql database. Each MySQL account is assigned a password, although what is stored in the Password column of the user table is not the plaintext version of the password, but a hash value computed from it. Password hash values are computed by the PASSWORD() function.

MySQL uses passwords in two phases of client/server communication:

When a client attempts to connect to the server, there is an initial authentication step in which the client must present a password that has a hash value matching the hash value stored in the user table for the account that the client wants to use.
After the client connects, it can (if it has sufficient privileges) set or change the password hashes for accounts listed in the user table. The client can do this by using the PASSWORD() function to generate a password hash, or by using the GRANT or SET PASSWORD statements.

In other words, the server uses hash values during authentication when a client first attempts to connect. The server generates hash values if a connected client invokes the PASSWORD() function or uses a GRANT or SET PASSWORD statement to set or change a password.

The password hashing mechanism was updated in MySQL 4.1 to provide better security and to reduce the risk of passwords being intercepted. However, this new mechanism is understood only by the 4.1 server and 4.1 clients, which can result in some compatibility problems. A 4.1 client can connect to a pre-4.1 server, because the client understands both the old and new password hashing mechanisms. However, a pre-4.1 client that attempts to connect to a 4.1 server may run into difficulties. For example, a 4.0 mysql client that attempts to connect to a 4.1 server may fail with the following error message:

shell> mysql -h localhost -u root
Client does not support authentication protocol requested
by server; consider upgrading MySQL client

Another common example of this phenomenon occurs for attempts to use the older PHP mysql extension after upgrading to MySQL 4.1 or newer. (See Section 17.3.1, “Common Problems with MySQL and PHP”.)

The following discussion describes the differences between the old and new password mechanisms, and what you should do if you upgrade your server to 4.1 but need to maintain backward compatibility with pre-4.1 clients. Additional information can be found in Section A.1.2.4, “Client does not support authentication protocol”. This information is of particular importance to PHP programmers migrating MySQL databases from version 4.0 or lower to version 4.1 or higher.

Note

This discussion contrasts 4.1 behavior with pre-4.1 behavior, but the 4.1 behavior described here actually begins with 4.1.1. MySQL 4.1.0 is an “odd” release because it has a slightly different mechanism than that implemented in 4.1.1 and up. Differences between 4.1.0 and more recent versions are described further in Section 5.5.8.2, “Password Hashing in MySQL 4.1.0”.

Prior to MySQL 4.1, password hashes computed by the PASSWORD() function are 16 bytes long. Such hashes look like this:

mysql> SELECT PASSWORD('mypass');
+--------------------+
| PASSWORD('mypass') |
+--------------------+
| 6f8c114b58f2ce9e   |
+--------------------+

The Password column of the user table (in which these hashes are stored) also is 16 bytes long before MySQL 4.1.

As of MySQL 4.1, the PASSWORD() function has been modified to produce a longer 41-byte hash value:

mysql> SELECT PASSWORD('mypass');
+-------------------------------------------+
| PASSWORD('mypass')                        |
+-------------------------------------------+
| *6C8989366EAF75BB670AD8EA7A7FC1176A95CEF4 |
+-------------------------------------------+

Accordingly, the Password column in the user table also must be 41 bytes long to store these values:

If you perform a new installation of MySQL 4.1, the Password column is made 41 bytes long automatically.
If you upgrade an older installation to 4.1, you should run the mysql_fix_privilege_tables script to increase the length of the Password column from 16 to 41 bytes. (The script does not change existing password values, which remain 16 bytes long.)

A widened Password column can store password hashes in both the old and new formats. The format of any given password hash value can be determined two ways:

The obvious difference is the length (16 bytes versus 41 bytes).
A second difference is that password hashes in the new format always begin with a “*” character, whereas passwords in the old format never do.

The longer password hash format has better cryptographic properties, and client authentication based on long hashes is more secure than that based on the older short hashes.

The differences between short and long password hashes are relevant both for how the server uses passwords during authentication and for how it generates password hashes for connected clients that perform password-changing operations.

The way in which the server uses password hashes during authentication is affected by the width of the Password column:

If the column is short, only short-hash authentication is used.
If the column is long, it can hold either short or long hashes, and the server can use either format:
- Pre-4.1 clients can connect, although because they know only about the old hashing mechanism, they can authenticate only using accounts that have short hashes.
- 4.1 clients can authenticate using accounts that have short or long hashes.

Even for short-hash accounts, the authentication process is actually a bit more secure for 4.1 and later clients than for older clients. In terms of security, the gradient from least to most secure is:

Pre-4.1 client authenticating with short password hash
4.1 client authenticating with short password hash
4.1 client authenticating with long password hash

The way in which the server generates password hashes for connected clients is affected by the width of the Password column and by the --old-passwords option. A 4.1 server generates long hashes only if certain conditions are met: The Password column must be wide enough to hold long values and the --old-passwords option must not be given. These conditions apply as follows:

The Password column must be wide enough to hold long hashes (41 bytes). If the column has not been updated and still has the pre-4.1 width of 16 bytes, the server notices that long hashes cannot fit into it and generates only short hashes when a client performs password-changing operations using PASSWORD(), GRANT, or SET PASSWORD. This is the behavior that occurs if you have upgraded to 4.1 but have not yet run the mysql_fix_privilege_tables script to widen the Password column.
If the Password column is wide, it can store either short or long password hashes. In this case, PASSWORD(), GRANT, and SET PASSWORD generate long hashes unless the server was started with the --old-passwords option. That option forces the server to generate short password hashes instead.

The purpose of the --old-passwords option is to enable you to maintain backward compatibility with pre-4.1 clients under circumstances where the server would otherwise generate long password hashes. The option does not affect authentication (4.1 clients can still use accounts that have long password hashes), but it does prevent creation of a long password hash in the user table as the result of a password-changing operation. Were that to occur, the account no longer could be used by pre-4.1 clients. Without the --old-passwords option, the following undesirable scenario is possible:

An old client connects to an account that has a short password hash.
The client changes its own password. Without --old-passwords, this results in the account having a long password hash.
The next time the old client attempts to connect to the account, it cannot, because the account has a long password hash that requires the new hashing mechanism during authentication. (Once an account has a long password hash in the user table, only 4.1 clients can authenticate for it, because pre-4.1 clients do not understand long hashes.)

This scenario illustrates that, if you must support older pre-4.1 clients, it is dangerous to run a 4.1 server without using the --old-passwords option. By running the server with --old-passwords, password-changing operations do not generate long password hashes and thus do not cause accounts to become inaccessible to older clients. (Those clients cannot inadvertently lock themselves out by changing their password and ending up with a long password hash.)

The downside of the --old-passwords option is that any passwords you create or change use short hashes, even for 4.1 clients. Thus, you lose the additional security provided by long password hashes. If you want to create an account that has a long hash (for example, for use by 4.1 clients), you must do so while running the server without --old-passwords.

The following scenarios are possible for running a 4.1 or later server:

Scenario 1: Short Password column in user table:

Only short hashes can be stored in the Password column.
The server uses only short hashes during client authentication.
For connected clients, password hash-generating operations involving PASSWORD(), GRANT, or SET PASSWORD use short hashes exclusively. Any change to an account's password results in that account having a short password hash.
The --old-passwords option can be used but is superfluous because with a short Password column, the server generates only short password hashes anyway.

Scenario 2: Long Password column; server not started with --old-passwords option:

Short or long hashes can be stored in the Password column.
4.1 and later clients can authenticate using accounts that have short or long hashes.
Pre-4.1 clients can authenticate only using accounts that have short hashes.
For connected clients, password hash-generating operations involving PASSWORD(), GRANT, or SET PASSWORD use long hashes exclusively. A change to an account's password results in that account having a long password hash.

As indicated earlier, a danger in this scenario is that it is possible for accounts that have a short password hash to become inaccessible to pre-4.1 clients. A change to such an account's password made via GRANT, PASSWORD(), or SET PASSWORD results in the account being given a long password hash. From that point on, no pre-4.1 client can authenticate to that account until the client upgrades to 4.1.

To deal with this problem, you can change a password in a special way. For example, normally you use SET PASSWORD as follows to change an account password:

SET PASSWORD FOR 'some_user'@'some_host' = PASSWORD('mypass');

To change the password but create a short hash, use the OLD_PASSWORD() function instead:

SET PASSWORD FOR 'some_user'@'some_host' = OLD_PASSWORD('mypass');

OLD_PASSWORD() is useful for situations in which you explicitly want to generate a short hash.

Scenario 3: Long Password column; server started with --old-passwords option:

Short or long hashes can be stored in the Password column.
4.1 clients can authenticate for accounts that have short or long hashes (but note that it is possible to create long hashes only when the server is started without --old-passwords).
Pre-4.1 clients can authenticate only for accounts that have short hashes.
For connected clients, password hash-generating operations involving PASSWORD(), GRANT, or SET PASSWORD use short hashes exclusively. Any change to an account's password results in that account having a short password hash.

In this scenario, you cannot create accounts that have long password hashes, because the --old-passwords option prevents generation of long hashes. Also, if you create an account with a long hash before using the --old-passwords option, changing the account's password while --old-passwords is in effect results in the account being given a short password, causing it to lose the security benefits of a longer hash.

The disadvantages for these scenarios may be summarized as follows:

In scenario 1, you cannot take advantage of longer hashes that provide more secure authentication.

In scenario 2, accounts with short hashes become inaccessible to pre-4.1 clients if you change their passwords without explicitly using OLD_PASSWORD().

In scenario 3, --old-passwords prevents accounts with short hashes from becoming inaccessible, but password-changing operations cause accounts with long hashes to revert to short hashes, and you cannot change them back to long hashes while --old-passwords is in effect.

5.5.8.1. Implications of Password Hashing Changes for Application Programs

An upgrade to MySQL 4.1 can cause a compatibility issue for applications that use PASSWORD() to generate passwords for their own purposes. Applications really should not do this, because PASSWORD() should be used only to manage passwords for MySQL accounts. But some applications use PASSWORD() for their own purposes anyway.

If you upgrade to 4.1 and run the server under conditions where it generates long password hashes, an application that uses PASSWORD() for its own passwords breaks. The recommended course of action is to modify the application to use another function, such as SHA1() or MD5(), to produce hashed values. If that is not possible, you can use the OLD_PASSWORD() function, which is provided to generate short hashes in the old format. But note that OLD_PASSWORD() may one day no longer be supported.

If the server is running under circumstances where it generates short hashes, OLD_PASSWORD() is available but is equivalent to PASSWORD().

PHP programmers migrating their MySQL databases from version 4.0 or lower to version 4.1 or higher should see Section 17.3, “MySQL PHP API”.

5.5.8.2. Password Hashing in MySQL 4.1.0

Password hashing in MySQL 4.1.0 differs from hashing in 4.1.1 and up. The 4.1.0 differences are:

Password hashes are 45 bytes long rather than 41 bytes.
The PASSWORD() function is non-repeatable. That is, with a given argument X, successive calls to PASSWORD(X) generate different results.

These differences make authentication in 4.1.0 incompatible with that of releases that follow it. If you have upgraded to MySQL 4.1.0, it is recommended that you upgrade to a newer version as soon as possible. After you do, reassign any long passwords in the user table so that they are compatible with the 41-byte format.

5.6. MySQL User Account Management

5.6.1. MySQL Usernames and Passwords
5.6.2. Adding New User Accounts to MySQL
5.6.3. Removing User Accounts from MySQL
5.6.4. Limiting Account Resources
5.6.5. Assigning Account Passwords
5.6.6. Keeping Your Password Secure
5.6.7. Using SSL for Secure Connections

This section describes how to set up accounts for clients of your MySQL server. It discusses the following topics:

The meaning of account names and passwords as used in MySQL and how that compares to names and passwords used by your operating system
How to set up new accounts and remove existing accounts
How to change passwords
Guidelines for using passwords securely
How to use secure connections with SSL

5.6.1. MySQL Usernames and Passwords

A MySQL account is defined in terms of a username and the client host or hosts from which the user can connect to the server. The account also has a password. There are several distinctions between the way usernames and passwords are used by MySQL and the way they are used by your operating system:

Usernames, as used by MySQL for authentication purposes, have nothing to do with usernames (login names) as used by Windows or Unix. On Unix, most MySQL clients by default try to log in using the current Unix username as the MySQL username, but that is for convenience only. The default can be overridden easily, because client programs allow any username to be specified with a -u or --user option. Because this means that anyone can attempt to connect to the server using any username, you cannot make a database secure in any way unless all MySQL accounts have passwords. Anyone who specifies a username for an account that has no password is able to connect successfully to the server.
MySQL usernames can be up to 16 characters long. This limit is hard-coded in the MySQL servers and clients, and trying to circumvent it by modifying the definitions of the tables in the mysql database does not work.
Note
You should never alter any of the tables in the mysql database in any manner whatsoever except by means of the procedure prescribed by MySQL AB that is described in Section 4.4.5, “mysql_fix_privilege_tables — Upgrade MySQL System Tables”. Attempting to redefine MySQL's system tables in any other fashion results in undefined (and unsupported!) behavior.
Operating system usernames are completely unrelated to MySQL usernames and may even be of a different maximum length. For example, Unix usernames typically are limited to eight characters.
MySQL usernames can be up to 16 characters long. Changing the maximum length is not supported. If you try to change it, for example by changing the length of the User column in the mysql database tables, this will result in unpredictable behavior. (Altering privilege tables is not supported in any case.) Operating system usernames might have a different maximum length. For example, Unix usernames typically are limited to eight characters.
MySQL passwords have nothing to do with passwords for logging in to your operating system. There is no necessary connection between the password you use to log in to a Windows or Unix machine and the password you use to access the MySQL server on that machine.
MySQL encrypts passwords using its own algorithm. This encryption is different from that used during the Unix login process. MySQL password encryption is the same as that implemented by the PASSWORD() SQL function. Unix password encryption is the same as that implemented by the ENCRYPT() SQL function. See the descriptions of the PASSWORD() and ENCRYPT() functions in Section 11.10.2, “Encryption and Compression Functions”. From version 4.1 on, MySQL employs a stronger authentication method that has better password protection during the connection process than in earlier versions. It is secure even if TCP/IP packets are sniffed or the mysql database is captured. (In earlier versions, even though passwords are stored in encrypted form in the user table, knowledge of the encrypted password value could be used to connect to the MySQL server.)

When you install MySQL, the grant tables are populated with an initial set of accounts. These accounts have names and access privileges that are described in Section 2.10.3, “Securing the Initial MySQL Accounts”, which also discusses how to assign passwords to them. Thereafter, you normally set up, modify, and remove MySQL accounts using statements such as GRANT and REVOKE. See Section 12.5.1, “Account Management Statements”.

When you connect to a MySQL server with a command-line client, you should specify the username and password for the account that you want to use:

shell> mysql --user=monty --password=guess db_name

If you prefer short options, the command looks like this:

shell> mysql -u monty -pguess db_name

There must be no space between the -p option and the following password value. See Section 4.2.2, “Connecting to the MySQL Server”.

The preceding commands include the password value on the command line, which can be a security risk. See Section 5.6.6, “Keeping Your Password Secure”. To avoid this problem, specify the --password or -p option without any following password value:

shell> mysql --user=monty --password db_name
shell> mysql -u monty -p db_name

When the password option has no password value, the client program prints a prompt and waits for you to enter the password. (In these examples, db_name is not interpreted as a password because it is separated from the preceding password option by a space.)

On some systems, the library routine that MySQL uses to prompt for a password automatically limits the password to eight characters. That is a problem with the system library, not with MySQL. Internally, MySQL does not have any limit for the length of the password. To work around the problem, change your MySQL password to a value that is eight or fewer characters long, or put your password in an option file.

5.6.2. Adding New User Accounts to MySQL

You can create MySQL accounts in two ways:

By using GRANT statements
By manipulating the MySQL grant tables directly with statements such as INSERT, UPDATE, or DELETE

The preferred method is to use GRANT statements, because they are more concise and less error-prone. GRANT is described in Section 12.5.1.2, “GRANT Syntax”.

Another option for creating accounts is to use one of several available third-party programs that offer capabilities for MySQL account administration. phpMyAdmin is one such program.

The following examples show how to use the mysql client program to set up new users. These examples assume that privileges are set up according to the defaults described in Section 2.10.3, “Securing the Initial MySQL Accounts”. This means that to make changes, you must connect to the MySQL server as the MySQL root user, and the root account must have the INSERT privilege for the mysql database and the RELOAD administrative privilege.

First, use the mysql program to connect to the server as the MySQL root user:

shell> mysql --user=root mysql

If you have assigned a password to the root account, you also need to supply a --password or -p option for this mysql command and also for those later in this section.

After connecting to the server as root, you can add new accounts. The following statements use GRANT to set up four new accounts:

mysql> GRANT ALL PRIVILEGES ON *.* TO 'monty'@'localhost'
    ->     IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
mysql> GRANT ALL PRIVILEGES ON *.* TO 'monty'@'%'
    ->     IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
mysql> GRANT RELOAD,PROCESS ON *.* TO 'admin'@'localhost';
mysql> GRANT USAGE ON *.* TO 'dummy'@'localhost';

The accounts created by these GRANT statements have the following properties:

Two of the accounts have a username of monty and a password of some_pass. Both accounts are superuser accounts with full privileges to do anything. One account ('monty'@'localhost') can be used only when connecting from the local host. The other ('monty'@'%') can be used to connect from any other host. Note that it is necessary to have both accounts for monty to be able to connect from anywhere as monty. Without the localhost account, the anonymous-user account for localhost that is created by mysql_install_db would take precedence when monty connects from the local host. As a result, monty would be treated as an anonymous user. The reason for this is that the anonymous-user account has a more specific Host column value than the 'monty'@'%' account and thus comes earlier in the user table sort order. (user table sorting is discussed in Section 5.5.4, “Access Control, Stage 1: Connection Verification”.)
One account has a username of admin and no password. This account can be used only by connecting from the local host. It is granted the RELOAD and PROCESS administrative privileges. These privileges allow the admin user to execute the mysqladmin reload, mysqladmin refresh, and mysqladmin flush-xxx commands, as well as mysqladmin processlist . No privileges are granted for accessing any databases. You could add such privileges later by issuing additional GRANT statements.
One account has a username of dummy and no password. This account can be used only by connecting from the local host. No privileges are granted. The USAGE privilege in the GRANT statement enables you to create an account without giving it any privileges. It has the effect of setting all the global privileges to 'N'. It is assumed that you will grant specific privileges to the account later.

As an alternative to GRANT, you can create the same accounts directly by issuing INSERT statements and then telling the server to reload the grant tables:

shell> mysql --user=root mysql
mysql> INSERT INTO user
    ->     VALUES('localhost','monty',PASSWORD('some_pass'),
    ->     'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO user
    ->     VALUES('%','monty',PASSWORD('some_pass'),
    ->     'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO user SET Host='localhost',User='admin',
    ->     Reload_priv='Y', Process_priv='Y';
mysql> INSERT INTO user (Host,User,Password)
    ->     VALUES('localhost','dummy','');
mysql> FLUSH PRIVILEGES;

The reason for using FLUSH PRIVILEGES when you create accounts with INSERT is to tell the server to re-read the grant tables. Otherwise, the changes go unnoticed until you restart the server. With GRANT, FLUSH PRIVILEGES is unnecessary.

The reason for using the PASSWORD() function with INSERT is to encrypt the password. The GRANT statement encrypts the password for you, so PASSWORD() is unnecessary.

The 'Y' values enable privileges for the accounts. Depending on your MySQL version, you may have to use a different number of 'Y' values in the first two INSERT statements. (Versions prior to 3.22.11 have fewer privilege columns, and versions from 4.0.2 on have more.) For the admin account, the more readable extended INSERT syntax using SET that is available starting with MySQL 3.22.11 is used.

In the INSERT statement for the dummy account, only the Host, User, and Password columns in the user table row are assigned values. None of the privilege columns are set explicitly, so MySQL assigns them all the default value of 'N'. This is equivalent to what GRANT USAGE does.

Note that to set up a superuser account, it is necessary only to create a user table entry with all of the privilege columns set to 'Y'. user table privileges are global, so no entries in any of the other grant tables are needed.

The next examples create three accounts and give them access to specific databases. Each of them has a username of custom and password of obscure.

To create the accounts with GRANT, use the following statements:

shell> mysql --user=root mysql
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
    ->     ON bankaccount.*
    ->     TO 'custom'@'localhost'
    ->     IDENTIFIED BY 'obscure';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
    ->     ON expenses.*
    ->     TO 'custom'@'whitehouse.gov'
    ->     IDENTIFIED BY 'obscure';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
    ->     ON customer.*
    ->     TO 'custom'@'server.domain'
    ->     IDENTIFIED BY 'obscure';

The three accounts can be used as follows:

The first account can access the bankaccount database, but only from the local host.
The second account can access the expenses database, but only from the host whitehouse.gov.
The third account can access the customer database, but only from the host server.domain.

To set up the custom accounts without GRANT, use INSERT statements as follows to modify the grant tables directly:

shell> mysql --user=root mysql
mysql> INSERT INTO user (Host,User,Password)
    ->     VALUES('localhost','custom',PASSWORD('obscure'));
mysql> INSERT INTO user (Host,User,Password)
    ->     VALUES('whitehouse.gov','custom',PASSWORD('obscure'));
mysql> INSERT INTO user (Host,User,Password)
    ->     VALUES('server.domain','custom',PASSWORD('obscure'));
mysql> INSERT INTO db
    ->     (Host,Db,User,Select_priv,Insert_priv,
    ->     Update_priv,Delete_priv,Create_priv,Drop_priv)
    ->     VALUES('localhost','bankaccount','custom',
    ->     'Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
    ->     (Host,Db,User,Select_priv,Insert_priv,
    ->     Update_priv,Delete_priv,Create_priv,Drop_priv)
    ->     VALUES('whitehouse.gov','expenses','custom',
    ->     'Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
    ->     (Host,Db,User,Select_priv,Insert_priv,
    ->     Update_priv,Delete_priv,Create_priv,Drop_priv)
    ->     VALUES('server.domain','customer','custom',
    ->     'Y','Y','Y','Y','Y','Y');
mysql> FLUSH PRIVILEGES;

The first three INSERT statements add user table entries that allow the user custom to connect from the various hosts with the given password, but grant no global privileges (all privileges are set to the default value of 'N'). The next three INSERT statements add db table entries that grant privileges to custom for the bankaccount, expenses, and customer databases, but only when accessed from the proper hosts. As usual when you modify the grant tables directly, you must tell the server to reload them with FLUSH PRIVILEGES so that the privilege changes take effect.

If you want to give a specific user access from all machines in a given domain (for example, mydomain.com), you can issue a GRANT statement that uses the “%” wildcard character in the host part of the account name:

mysql> GRANT ...
    ->     ON *.*
    ->     TO 'myname'@'%.mydomain.com'
    ->     IDENTIFIED BY 'mypass';

To do the same thing by modifying the grant tables directly, do this:

mysql> INSERT INTO user (Host,User,Password,...)
    ->     VALUES('%.mydomain.com','myname',PASSWORD('mypass'),...);
mysql> FLUSH PRIVILEGES;

5.6.3. Removing User Accounts from MySQL

To remove an account, use the DROP USER statement, which was added in MySQL 4.1.1. For older versions of MySQL, use DELETE instead. The account removal procedure is described in Section 12.5.1.1, “DROP USER Syntax”.

5.6.4. Limiting Account Resources

Before MySQL 4.0.2, the only available method for limiting use of MySQL server resources is to set the max_user_connections system variable to a non-zero value. But that method is strictly global. It does not allow for management of individual accounts. Also, it limits only the number of simultaneous connections made using a single account, not what a client can do once connected. Both types of control are of interest to many MySQL administrators, particularly those for Internet Service Providers.

Starting from MySQL 4.0.2, you can limit the following server resources for individual accounts:

The number of queries that an account can issue per hour
The number of updates that an account can issue per hour
The number of times an account can connect to the server per hour

Any statement that a client can issue counts against the query limit. Only statements that modify databases or tables count against the update limit.

An “account” in this context is assessed against the actual host from which a user connects. Suppose that there is a row in the user table that has User and Host values of usera and %.example.com, to allow usera to connect from any host in the example.com domain. If usera connects simultaneously from host1.example.com and host2.example.com, the server applies the account resource limits separately to each connection. If usera connects again from host1.example.com, the server applies the limits for that connection together with the existing connection from that host.

As a prerequisite for using this feature, the user table in the mysql database must contain the resource-related columns. Resource limits are stored in the max_questions, max_updates, max_connections, and max_user_connections columns. If your user table does not have these columns, it must be upgraded; see Section 4.4.5, “mysql_fix_privilege_tables — Upgrade MySQL System Tables”.

To set resource limits with a GRANT statement, use a WITH clause that names each resource to be limited and a per-hour count indicating the limit value. For example, to create a new account that can access the customer database, but only in a limited fashion, issue this statement:

mysql> GRANT ALL ON customer.* TO 'francis'@'localhost'
    ->     IDENTIFIED BY 'frank'
    ->     WITH MAX_QUERIES_PER_HOUR 20
    ->          MAX_UPDATES_PER_HOUR 10
    ->          MAX_CONNECTIONS_PER_HOUR 5;

The limit types need not all be named in the WITH clause, but those named can be present in any order. The value for each per-hour limit should be an integer representing a count per hour. If the GRANT statement has no WITH clause, the limits are each set to the default value of zero (that is, no limit).

To set or change limits for an existing account, use a GRANT USAGE statement at the global level (ON *.*). The following statement changes the query limit for francis to 100:

mysql> GRANT USAGE ON *.* TO 'francis'@'localhost'
    ->     WITH MAX_QUERIES_PER_HOUR 100;

This statement leaves the account's existing privileges unchanged and modifies only the limit values specified.

To remove an existing limit, set its value to zero. For example, to remove the limit on how many times per hour francis can connect, use this statement:

mysql> GRANT USAGE ON *.* TO 'francis'@'localhost'
    ->     WITH MAX_CONNECTIONS_PER_HOUR 0;

Resource-use counting takes place when any account has a non-zero limit placed on its use of any of the resources.

As the server runs, it counts the number of times each account uses resources. If an account reaches its limit on number of connections within the last hour, further connections for the account are rejected until that hour is up. Similarly, if the account reaches its limit on the number of queries or updates, further queries or updates are rejected until the hour is up. In all such cases, an appropriate error message is issued.

Queries for which results are served from the query cache do not count against the MAX_QUERIES_PER_HOUR limit.

The current per-hour resource-use counts can be reset globally for all accounts, or individually for a given account:

To reset the current counts to zero for all accounts, issue a FLUSH USER_RESOURCES statement. The counts also can be reset by reloading the grant tables (for example, with a FLUSH PRIVILEGES statement or a mysqladmin reload command).
The counts for an individual account can be set to zero by re-granting it any of its limits. To do this, use GRANT USAGE as described earlier and specify a limit value equal to the value that the account currently has.

Counter resets do not affect the MAX_USER_CONNECTIONS limit.

All counts begin at zero when the server starts; counts are not carried over through a restart.

5.6.5. Assigning Account Passwords

Passwords may be assigned from the command line by using the mysqladmin command:

shell> mysqladmin -u user_name -h host_name password "newpwd"

The account for which this command resets the password is the one with a user table row that matches user_name in the User column and the client host from which you connect in the Host column.

Another way to assign a password to an account is to issue a SET PASSWORD statement:

mysql> SET PASSWORD FOR 'jeffrey'@'%' = PASSWORD('biscuit');

Only users such as root that have update access to the mysql database can change the password for other users. If you are not connected as an anonymous user, you can change your own password by omitting the FOR clause:

mysql> SET PASSWORD = PASSWORD('biscuit');

You can also use a GRANT USAGE statement at the global level (ON *.*) to assign a password to an account without affecting the account's current privileges:

mysql> GRANT USAGE ON *.* TO 'jeffrey'@'%' IDENTIFIED BY 'biscuit';

Although it is generally preferable to assign passwords using one of the preceding methods, you can also do so by modifying the user table directly:

To establish a password when creating a new account, provide a value for the Password column:

shell> mysql -u root mysql
mysql> INSERT INTO user (Host,User,Password)
    -> VALUES('%','jeffrey',PASSWORD('biscuit'));
mysql> FLUSH PRIVILEGES;

To change the password for an existing account, use UPDATE to set the Password column value:

shell> mysql -u root mysql
mysql> UPDATE user SET Password = PASSWORD('bagel')
    -> WHERE Host = '%' AND User = 'francis';
mysql> FLUSH PRIVILEGES;

When you assign an account a non-empty password using SET PASSWORD, INSERT, or UPDATE, you must use the PASSWORD() function to encrypt it. PASSWORD() is necessary because the user table stores passwords in encrypted form, not as plaintext. If you forget that fact, you are likely to set passwords like this:

shell> mysql -u root mysql
mysql> INSERT INTO user (Host,User,Password)
    -> VALUES('%','jeffrey','biscuit');
mysql> FLUSH PRIVILEGES;

The result is that the literal value 'biscuit' is stored as the password in the user table, not the encrypted value. When jeffrey attempts to connect to the server using this password, the value is encrypted and compared to the value stored in the user table. However, the stored value is the literal string 'biscuit', so the comparison fails and the server rejects the connection:

shell> mysql -u jeffrey -pbiscuit test
Access denied

If you assign passwords using the GRANT ... IDENTIFIED BY statement or the mysqladmin password command, they both take care of encrypting the password for you. The PASSWORD() function is unnecessary.

Note

PASSWORD() encryption is different from Unix password encryption. See Section 5.6.1, “MySQL Usernames and Passwords”.

5.6.6. Keeping Your Password Secure

On an administrative level, you should never grant access to the user grant table to any non-administrative accounts. Passwords in the user table are stored in encrypted form, but in versions of MySQL earlier than 4.1, knowing the encrypted password for an account makes it possible to connect to the server using that account.

When you run a client program to connect to the MySQL server, it is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed here, along with an assessment of the risks of each method:

Use a -pyour_pass or --password=your_pass option on the command line. For example:
```
shell> mysql -u francis -pfrank db_name
```
This is convenient but insecure, because your password becomes visible to system status programs such as ps that may be invoked by other users to display command lines. MySQL clients typically overwrite the command-line password argument with zeros during their initialization sequence. However, there is still a brief interval during which the value is visible. On some systems this strategy is ineffective, anyway, and the password remains visible to ps. (SystemV Unix systems and perhaps others are subject to this problem.)
Use the -p or --password option with no password value specified. In this case, the client program solicits the password from the terminal:
```
shell> mysql -u francis -p db_name
Enter password: ********
```
The “*” characters indicate where you enter your password. The password is not displayed as you enter it.
It is more secure to enter your password this way than to specify it on the command line because it is not visible to other users. However, this method of entering a password is suitable only for programs that you run interactively. If you want to invoke a client from a script that runs non-interactively, there is no opportunity to enter the password from the terminal. On some systems, you may even find that the first line of your script is read and interpreted (incorrectly) as your password.
Store your password in an option file. For example, on Unix you can list your password in the [client] section of the .my.cnf file in your home directory:
```
[client]
password=your_pass
```
If you store your password in .my.cnf, the file should not be accessible to anyone but yourself. To ensure this, set the file access mode to 400 or 600. For example:
```
shell> chmod 600 .my.cnf
```
Section 4.2.3.2, “Using Option Files”, discusses option files in more detail.
Store your password in the MYSQL_PWD environment variable. This method of specifying your MySQL password must be considered extremely insecure and should not be used. Some versions of ps include an option to display the environment of running processes. If you set MYSQL_PWD, your password is exposed to any other user who runs ps. Even on systems without such a version of ps, it is unwise to assume that there are no other methods by which users can examine process environments. See Section 2.14, “Environment Variables”.

All in all, the safest methods are to have the client program prompt for the password or to specify the password in a properly protected option file.

5.6.7. Using SSL for Secure Connections

5.6.7.1. Basic SSL Concepts
5.6.7.2. Using SSL Connections
5.6.7.3. SSL Command Options
5.6.7.4. Setting Up SSL Certificates for MySQL
5.6.7.5. Connecting to MySQL Remotely from Windows with SSH

Beginning with version 4.0.0, MySQL has support for secure (encrypted) connections between MySQL clients and the server using the Secure Sockets Layer (SSL) protocol. This section discusses how to use SSL connections. For information on how to require users to use SSL connections, see the discussion of the REQUIRE clause of the GRANT statement in Section 12.5.1.2, “GRANT Syntax”.

The standard configuration of MySQL is intended to be as fast as possible, so encrypted connections are not used by default. Doing so would make the client/server protocol much slower. Encrypting data is a CPU-intensive operation that requires the computer to do additional work and can delay other MySQL tasks. For applications that require the security provided by encrypted connections, the extra computation is warranted.

MySQL allows encryption to be enabled on a per-connection basis. You can choose a normal unencrypted connection or a secure encrypted SSL connection according the requirements of individual applications.

Secure connections are based on the OpenSSL API and are available through the MySQL C API. Replication uses the C API, so secure connections can be used between master and slave servers.

5.6.7.1. Basic SSL Concepts

To understand how MySQL uses SSL, it is necessary to explain some basic SSL and X509 concepts. People who are familiar with these can skip this part of the discussion.

By default, MySQL uses unencrypted connections between the client and the server. This means that someone with access to the network could watch all your traffic and look at the data being sent or received. They could even change the data while it is in transit between client and server. To improve security a little, you can compress client/server traffic by using the --compress option when invoking client programs. However, this does not foil a determined attacker.

When you need to move information over a network in a secure fashion, an unencrypted connection is unacceptable. Encryption is the way to make any kind of data unreadable. In fact, today's practice requires many additional security elements from encryption algorithms. They should resist many kind of known attacks such as changing the order of encrypted messages or replaying data twice.

SSL is a protocol that uses different encryption algorithms to ensure that data received over a public network can be trusted. It has mechanisms to detect any data change, loss, or replay. SSL also incorporates algorithms that provide identity verification using the X509 standard.

X509 makes it possible to identify someone on the Internet. It is most commonly used in e-commerce applications. In basic terms, there should be some company called a “Certificate Authority” (or CA) that assigns electronic certificates to anyone who needs them. Certificates rely on asymmetric encryption algorithms that have two encryption keys (a public key and a secret key). A certificate owner can show the certificate to another party as proof of identity. A certificate consists of its owner's public key. Any data encrypted with this public key can be decrypted only using the corresponding secret key, which is held by the owner of the certificate.

If you need more information about SSL, X509, or encryption, use your favorite Internet search engine to search for the keywords in which you are interested.

5.6.7.2. Using SSL Connections

To use SSL connections between the MySQL server and client programs, your system must support OpenSSL and your version of MySQL must be 4.0.0 or newer and built with SSL support.

To get secure connections to work with MySQL and SSL, you must do the following:

Install the OpenSSL library if it has not already been installed. We have tested MySQL with OpenSSL 0.9.6. To obtain OpenSSL, visit http://www.openssl.org.
If you are not using a binary (precompiled) version of MySQL that has been built with SSL support, configure a MySQL source distribution to use SSL. When you configure MySQL, invoke the configure script with the --with-vio and --with-openssl options:
```
shell> ./configure --with-vio --with-openssl
```
Make sure that you have upgraded your grant tables to include the SSL-related columns in the mysql.user table. This is necessary if your grant tables date from a version of MySQL older than 4.0. The upgrade procedure is described in Section 4.4.5, “mysql_fix_privilege_tables — Upgrade MySQL System Tables”.
To check whether a server binary is compiled with SSL support, invoke it with the --ssl option. An error will occur if the server does not support SSL:
```
shell> mysqld --ssl --help
060525 14:18:52 [ERROR] mysqld: unknown option '--ssl'
```
To check whether a running mysqld server supports SSL, examine the value of the have_openssl system variable:
```
mysql> SHOW VARIABLES LIKE 'have_openssl';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| have_openssl  | YES   |
+---------------+-------+
```
If the value is YES, the server supports OpenSSL connections.

To enable SSL connections, the proper SSL-related command options must be used (see Section 5.6.7.3, “SSL Command Options”).

To start the MySQL server so that it allows clients to connect via SSL, use the options that identify the key and certificate files the server needs when establishing a secure connection:

shell> mysqld --ssl-ca=cacert.pem \
       --ssl-cert=server-cert.pem \
       --ssl-key=server-key.pem

--ssl-ca identifies the Certificate Authority (CA) certificate.
--ssl-cert identifies the server public key. This can be sent to the client and authenticated against the CA certificate that it has.
--ssl-key identifies the server private key.

To establish a secure connection to a MySQL server with SSL support, the options that a client must specify depend on the SSL requirements of the user account that the client uses. (See the discussion of the REQUIRE clause in Section 12.5.1.2, “GRANT Syntax”.)

If the account has no special SSL requirements or was created using a GRANT statement that includes the REQUIRE SSL option, a client can connect securely by using just the --ssl-ca option:

shell> mysql --ssl-ca=cacert.pem

To require that a client certificate also be specified, create the account using the REQUIRE X509 option. Then the client must also specify the proper client key and certificate files or the server will reject the connection:

shell> mysql --ssl-ca=cacert.pem \
       --ssl-cert=client-cert.pem \
       --ssl-key=client-key.pem

In other words, the options are similar to those used for the server. Note that the Certificate Authority certificate has to be the same.

A client can determine whether the current connection with the server uses SSL by checking the value of the Ssl_cipher status variable. The value of Ssl_cipher is non-empty if SSL is used, and empty otherwise. For example:

mysql> SHOW STATUS LIKE 'Ssl_cipher';
+---------------+--------------------+
| Variable_name | Value              |
+---------------+--------------------+
| Ssl_cipher    | DHE-RSA-AES256-SHA |
+---------------+--------------------+

For the mysql client, you can use the STATUS or \s command and check the SSL line:

mysql> \s
...
SSL:                    Not in use
...

Or:

mysql> \s
...
SSL:                    Cipher in use is DHE-RSA-AES256-SHA
...

To establish a secure connection from within an application program, use the mysql_ssl_set() C API function to set the appropriate certificate options before calling mysql_real_connect(). See Section 17.2.3.65, “mysql_ssl_set()”.

5.6.7.3. SSL Command Options

The following list describes options that are used for specifying the use of SSL, certificate files, and key files. These options are available beginning with MySQL 4.0. They can be given on the command line or in an option file. These options are not available unless MySQL has been built with SSL support. See Section 5.6.7.2, “Using SSL Connections”. (There are also --master-ssl* options that can be used for setting up a secure connection from a slave replication server to a master server; see Section 14.8, “Replication Startup Options”.)

Name	Cmd-Line	Option file	System Var	Var Scope	Dynamic
have_openssl			Yes	Global	No
skip-ssl	Yes	Yes
ssl	Yes	Yes
ssl-ca	Yes	Yes		Global	No
- Variable: ssl_ca			Yes	Global	No
ssl-capath	Yes	Yes		Global	No
- Variable: ssl_capath			Yes	Global	No
ssl-cert	Yes	Yes		Global	No
- Variable: ssl_cert			Yes	Global	No
ssl-cipher	Yes	Yes		Global	No
- Variable: ssl_cipher			Yes	Global	No
ssl-key	Yes	Yes		Global	No
- Variable: ssl_key			Yes	Global	No

--ssl
For the server, this option specifies that the server allows SSL connections. For a client program, it allows the client to connect to the server using SSL. This option is not sufficient in itself to cause an SSL connection to be used. You must also specify the --ssl-ca option, and possibly the --ssl-cert and --ssl-key options.
This option is more often used in its opposite form to override any other SSL options and indicate that SSL should not be used. To do this, specify the option as --skip-ssl or --ssl=0.
Note that use of --ssl does not require an SSL connection. For example, if the server or client is compiled without SSL support, a normal unencrypted connection is used.
The secure way to require use of an SSL connection is to create an account on the server that includes a REQUIRE SSL clause in the GRANT statement. Then use that account to connect to the server, where both the server and the client have SSL support enabled.
The REQUIRE clause allows other SSL-related restrictions as well. The description of REQUIRE in Section 12.5.1.2, “GRANT Syntax”, provides additional detail about which SSL command options may or must be specified by clients that connect using accounts that are created using the various REQUIRE options.
--ssl-ca=file_name
The path to a file that contains a list of trusted SSL CAs.
--ssl-capath=directory_name
The path to a directory that contains trusted SSL CA certificates in PEM format.
--ssl-cert=file_name
The name of the SSL certificate file to use for establishing a secure connection.
--ssl-cipher=cipher_list
A list of allowable ciphers to use for SSL encryption. For greatest portability, cipher_list should be a list of one or more cipher names, separated by colons. Examples:
```
--ssl-cipher=AES128-SHA
--ssl-cipher=DHE-RSA-AES256-SHA:AES128-SHA
```
This format is understood both by OpenSSL and yaSSL. OpenSSL supports a more flexible syntax for specifying ciphers, as described in the OpenSSL documentation at http://www.openssl.org/docs/apps/ciphers.html. However, this extended syntax will fail if used with a MySQL installation compiled against yaSSL (which may be the case for MySQL 5.0 and up).
If no cipher in the list is supported, SSL connections will not work.
--ssl-key=file_name
The name of the SSL key file to use for establishing a secure connection.

5.6.7.4. Setting Up SSL Certificates for MySQL

This section demonstrates how to set up SSL certificate and key files for use by MySQL servers and clients. The first example shows a simplified procedure such as you might use from the command line. The second shows a script that contains more detail. Both examples use the openssl command that is part of OpenSSL.

The following example shows a set of commands to create MySQL server and client certificate and key files. You will need to respond to several prompts by the openssl commands. For testing, you can press Enter to all prompts. For production use, you should provide non-empty responses.

# Create clean environment
shell> rm -rf newcerts
shell> mkdir newcerts && cd newcerts

# Create CA certificate
shell> openssl genrsa 2048 > ca-key.pem
shell> openssl req -new -x509 -nodes -days 1000 \
         -key ca-key.pem > ca-cert.pem

# Create server certificate
shell> openssl req -newkey rsa:2048 -days 1000 \
         -nodes -keyout server-key.pem > server-req.pem
shell> openssl x509 -req -in server-req.pem -days 1000 \
         -CA ca-cert.pem -CAkey ca-key.pem -set_serial 01 > server-cert.pem

# Create client certificate
shell> openssl req -newkey rsa:2048 -days 1000 \
         -nodes -keyout client-key.pem > client-req.pem
shell> openssl x509 -req -in client-req.pem -days 1000 \
         -CA ca-cert.pem -CAkey ca-key.pem -set_serial 01 > client-cert.pem

Here is an example script that shows how to set up SSL certificates for MySQL:

DIR=`pwd`/openssl
PRIV=$DIR/private

mkdir $DIR $PRIV $DIR/newcerts
cp /usr/share/ssl/openssl.cnf $DIR
replace ./demoCA $DIR -- $DIR/openssl.cnf

# Create necessary files: $database, $serial and $new_certs_dir
# directory (optional)

touch $DIR/index.txt
echo "01" > $DIR/serial

#
# Generation of Certificate Authority(CA)
#

openssl req -new -x509 -keyout $PRIV/cakey.pem -out $DIR/cacert.pem \
    -config $DIR/openssl.cnf

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# ................++++++
# .........++++++
# writing new private key to '/home/monty/openssl/private/cakey.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL admin
# Email Address []:

#
# Create server request and key
#
openssl req -new -keyout $DIR/server-key.pem -out \
    $DIR/server-req.pem -days 3600 -config $DIR/openssl.cnf

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# ..++++++
# ..........++++++
# writing new private key to '/home/monty/openssl/server-key.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL server
# Email Address []:
#
# Please enter the following 'extra' attributes
# to be sent with your certificate request
# A challenge password []:
# An optional company name []:

#
# Remove the passphrase from the key
#
openssl rsa -in $DIR/server-key.pem -out $DIR/server-key.pem

#
# Sign server cert
#
openssl ca  -policy policy_anything -out $DIR/server-cert.pem \
    -config $DIR/openssl.cnf -infiles $DIR/server-req.pem

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Enter PEM pass phrase:
# Check that the request matches the signature
# Signature ok
# The Subjects Distinguished Name is as follows
# countryName           :PRINTABLE:'FI'
# organizationName      :PRINTABLE:'MySQL AB'
# commonName            :PRINTABLE:'MySQL admin'
# Certificate is to be certified until Sep 13 14:22:46 2003 GMT
# (365 days)
# Sign the certificate? [y/n]:y
#
#
# 1 out of 1 certificate requests certified, commit? [y/n]y
# Write out database with 1 new entries
# Data Base Updated

#
# Create client request and key
#
openssl req -new -keyout $DIR/client-key.pem -out \
    $DIR/client-req.pem -days 3600 -config $DIR/openssl.cnf

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# .....................................++++++
# .............................................++++++
# writing new private key to '/home/monty/openssl/client-key.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL user
# Email Address []:
#
# Please enter the following 'extra' attributes
# to be sent with your certificate request
# A challenge password []:
# An optional company name []:

#
# Remove the passphrase from the key
#
openssl rsa -in $DIR/client-key.pem -out $DIR/client-key.pem

#
# Sign client cert
#

openssl ca  -policy policy_anything -out $DIR/client-cert.pem \
    -config $DIR/openssl.cnf -infiles $DIR/client-req.pem

# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Enter PEM pass phrase:
# Check that the request matches the signature
# Signature ok
# The Subjects Distinguished Name is as follows
# countryName           :PRINTABLE:'FI'
# organizationName      :PRINTABLE:'MySQL AB'
# commonName            :PRINTABLE:'MySQL user'
# Certificate is to be certified until Sep 13 16:45:17 2003 GMT
# (365 days)
# Sign the certificate? [y/n]:y
#
#
# 1 out of 1 certificate requests certified, commit? [y/n]y
# Write out database with 1 new entries
# Data Base Updated

#
# Create a my.cnf file that you can use to test the certificates
#

cnf=""
cnf="$cnf [client]"
cnf="$cnf ssl-ca=$DIR/cacert.pem"
cnf="$cnf ssl-cert=$DIR/client-cert.pem"
cnf="$cnf ssl-key=$DIR/client-key.pem"
cnf="$cnf [mysqld]"
cnf="$cnf ssl-ca=$DIR/cacert.pem"
cnf="$cnf ssl-cert=$DIR/server-cert.pem"
cnf="$cnf ssl-key=$DIR/server-key.pem"
echo $cnf | replace " " '
' > $DIR/my.cnf

To test SSL connections, start the server as follows, where $DIR is the pathname to the directory where the sample my.cnf option file is located:

shell> mysqld --defaults-file=$DIR/my.cnf &

Then invoke a client program using the same option file:

shell> mysql --defaults-file=$DIR/my.cnf

If you have a MySQL source distribution, you can also test your setup by modifying the preceding my.cnf file to refer to the demonstration certificate and key files in the SSL directory of the distribution.

5.6.7.5. Connecting to MySQL Remotely from Windows with SSH

Here is a note that describes how to get a secure connection to a remote MySQL server with SSH (by David Carlson <[email protected]>):

Install an SSH client on your Windows machine. As a user, the best non-free one I have found is from SecureCRT from http://www.vandyke.com/. Another option is f-secure from http://www.f-secure.com/. You can also find some free ones on Google at http://directory.google.com/Top/Computers/Internet/Protocols/SSH/Clients/Windows/.
Start your Windows SSH client. Set Host_Name = yourmysqlserver_URL_or_IP. Set userid=your_userid to log in to your server. This userid value might not be the same as the username of your MySQL account.
Set up port forwarding. Either do a remote forward (Set local_port: 3306, remote_host: yourmysqlservername_or_ip, remote_port: 3306 ) or a local forward (Set port: 3306, host: localhost, remote port: 3306).
Save everything, otherwise you will have to redo it the next time.
Log in to your server with the SSH session you just created.
On your Windows machine, start some ODBC application (such as Access).
Create a new file in Windows and link to MySQL using the ODBC driver the same way you normally do, except type in localhost for the MySQL host server, not yourmysqlservername.

At this point, you should have an ODBC connection to MySQL, encrypted using SSH.

5.7. Running Multiple MySQL Servers on the Same Machine

5.7.1. Running Multiple Servers on Windows
5.7.2. Running Multiple Servers on Unix
5.7.3. Using Client Programs in a Multiple-Server Environment

In some cases, you might want to run multiple mysqld servers on the same machine. You might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you might want to give different users access to different mysqld servers that they manage themselves. (For example, you might be an Internet Service Provider that wants to provide independent MySQL installations for different customers.)

To run multiple servers on a single machine, each server must have unique values for several operating parameters. These can be set on the command line or in option files. See Section 4.2.3, “Specifying Program Options”.

At least the following options must be different for each server:

--port=port_num
--port controls the port number for TCP/IP connections. (Alternatively, if the host has multiple network addresses, you can use --bind-adress to cause different servers to listen to different interfaces.)
--socket=path
--socket controls the Unix socket file path on Unix and the name of the named pipe on Windows. On Windows, it is necessary to specify distinct pipe names only for those servers that support named-pipe connections.
--shared-memory-base-name=name
The name of shared memory to use for shared-memory connections. This option is available only on Windows. The default name is MYSQL. The name is case sensitive. This option was added in MySQL 4.1.
--pid-file=file_name
This option is used only on Unix. It indicates the pathname of the file in which the server writes its process ID.

If you use the following log file options, they must be different for each server:

--log=file_name
--log-bin=file_name
--log-update=file_name
--log-error=file_name
--log-isam=file_name
--bdb-logdir=file_name

Section 5.3.6, “Server Log Maintenance”, discusses the log file options further.

For better performance, you can specify the following options differently for each server, to spread the load between several physical disks:

--tmpdir=path
--bdb-tmpdir=path

Having different temporary directories is also recommended to make it easier to determine which MySQL server created any given temporary file.

With very limited exceptions, each server should use a different data directory, which is specified using the --datadir=path option.

Warning

Normally, you should never have two servers that update data in the same databases. This may lead to unpleasant surprises if your operating system does not support fault-free system locking. If (despite this warning) you run multiple servers using the same data directory and they have logging enabled, you must use the appropriate options to specify log filenames that are unique to each server. Otherwise, the servers try to log to the same files. Please note that this kind of setup only works with ISAM, MyISAM and MERGE tables, and not with any of the other storage engines.

The warning against sharing a data directory among servers also applies in an NFS environment. Allowing multiple MySQL servers to access a common data directory over NFS is a very bad idea.

The primary problem is that NFS is the speed bottleneck. It is not meant for such use.
Another risk with NFS is that you must devise a way to ensure that two or more servers do not interfere with each other. Usually NFS file locking is handled by the lockd daemon, but at the moment there is no platform that performs locking 100% reliably in every situation.

Make it easy for yourself: Forget about sharing a data directory among servers over NFS. A better solution is to have one computer that contains several CPUs and use an operating system that handles threads efficiently.

If you have multiple MySQL installations in different locations, you can specify the base installation directory for each server with the --basedir=path option to cause each server to use a different data directory, log files, and PID file. (The defaults for all these values are determined relative to the base directory). In that case, the only other options you need to specify are the --socket and --port options. For example, suppose that you install different versions of MySQL using tarfile binary distributions. These install in different locations, so you can start the server for each installation using bin/mysqld_safe under its own corresponding base directory. mysqld_safe determines the proper --basedir option to pass to mysqld, and you need specify only the --socket and --port options to mysqld_safe. (For versions of MySQL older than 4.0, use safe_mysqld rather than mysqld_safe.)

As discussed in the following sections, it is possible to start additional servers by setting environment variables or by specifying appropriate command-line options. However, if you need to run multiple servers on a more permanent basis, it is more convenient to use option files to specify for each server those option values that must be unique to it. The --defaults-file option is useful for this purpose.

5.7.1. Running Multiple Servers on Windows

5.7.1.1. Starting Multiple Windows Servers at the Command Line
5.7.1.2. Starting Multiple Windows Servers as Services

You can run multiple servers on Windows by starting them manually from the command line, each with appropriate operating parameters. On Windows NT-based systems, you also have the option of installing several servers as Windows services and running them that way. General instructions for running MySQL servers from the command line or as services are given in Section 2.3, “Installing MySQL on Windows”. This section describes how to make sure that you start each server with different values for those startup options that must be unique per server, such as the data directory. These options are described in Section 5.7, “Running Multiple MySQL Servers on the Same Machine”.

5.7.1.1. Starting Multiple Windows Servers at the Command Line

To start multiple servers manually from the command line, you can specify the appropriate options on the command line or in an option file. It is more convenient to place the options in an option file, but it is necessary to make sure that each server gets its own set of options. To do this, create an option file for each server and tell the server the filename with a --defaults-file option when you run it.

Suppose that you want to run mysqld on port 3307 with a data directory of C:\mydata1, and mysqld-max on port 3308 with a data directory of C:\mydata2. (To do this, make sure that before you start the servers, each data directory exists and has its own copy of the mysql database that contains the grant tables.) Then create two option files. For example, create one file named C:\my-opts1.cnf that looks like this:

[mysqld]
datadir = C:/mydata1
port = 3307

Create a second file named C:\my-opts2.cnf that looks like this:

[mysqld]
datadir = C:/mydata2
port = 3308

Then start each server with its own option file:

C:\> C:\mysql\bin\mysqld --defaults-file=C:\my-opts1.cnf
C:\> C:\mysql\bin\mysqld-max --defaults-file=C:\my-opts2.cnf

On NT, each server starts in the foreground (no new prompt appears until the server exits later), so you will need to issue those two commands in separate console windows.

To shut down the servers, you must connect to each using the appropriate port number:

C:\> C:\mysql\bin\mysqladmin --port=3307 shutdown
C:\> C:\mysql\bin\mysqladmin --port=3308 shutdown

Servers configured as just described allow clients to connect over TCP/IP. If your version of Windows supports named pipes and you also want to allow named-pipe connections, use the mysqld-nt or mysqld-max-nt servers and specify options that enable the named pipe and specify its name. Each server that supports named-pipe connections must use a unique pipe name. For example, the C:\my-opts1.cnf file might be written like this:

[mysqld]
datadir = C:/mydata1
port = 3307
enable-named-pipe
socket = mypipe1

Then start the server this way:

C:\> C:\mysql\bin\mysqld-nt --defaults-file=C:\my-opts1.cnf

Modify C:\my-opts2.cnf similarly for use by the second server.

A similar procedure applies for servers that you want to support shared-memory connections. Enable such connections with the --shared-memory option and specify a unique shared-memory name for each server with the --shared-memory-base-name option.

5.7.1.2. Starting Multiple Windows Servers as Services

On NT-based systems, a MySQL server can run as a Windows service. The procedures for installing, controlling, and removing a single MySQL service are described in Section 2.3.11, “Starting MySQL as a Windows Service”.

As of MySQL 4.0.2, you can install multiple servers as services. In this case, you must make sure that each server uses a different service name in addition to all the other parameters that must be unique per server.

For the following instructions, assume that you want to run the mysqld-nt server from two different versions of MySQL that are installed at C:\mysql-4.0.8 and C:\mysql-4.0.17, respectively. (This might be the case if you are running 4.0.8 as your production server, but want to test 4.0.17 before upgrading to it.)

The following principles apply when installing a MySQL service with the --install or --install-manual option:

If you specify no service name, the server uses the default service name of MySQL and the server reads options from the [mysqld] group in the standard option files.
If you specify a service name after the --install option, the server ignores the [mysqld] option group and instead reads options from the group that has the same name as the service. The server reads options from the standard option files.
If you specify a --defaults-file option after the service name, the server ignores the standard option files and reads options only from the [mysqld] group of the named file.

Note

Before MySQL 4.0.17, only a server installed using the default service name (MySQL) or one installed explicitly with a service name of mysqld will read the [mysqld] group in the standard option files. As of 4.0.17, all servers read the [mysqld] group if they read the standard option files, even if they are installed using another service name. This allows you to use the [mysqld] group for options that should be used by all MySQL services, and an option group named after each service for use by the server installed with that service name.

Based on the preceding information, you have several ways to set up multiple services. The following instructions describe some examples. Before trying any of them, be sure that you shut down and remove any existing MySQL services first.

Approach 1: Specify the options for all services in one of the standard option files. To do this, use a different service name for each server. Suppose that you want to run the 4.0.8 mysqld-nt using the service name of mysqld1 and the 4.0.17 mysqld-nt using the service name mysqld2. In this case, you can use the [mysqld1] group for 4.0.8 and the [mysqld2] group for 4.0.17. For example, you can set up C:\my.cnf like this:
```
# options for mysqld1 service
[mysqld1]
basedir = C:/mysql-4.0.8
port = 3307
enable-named-pipe
socket = mypipe1

# options for mysqld2 service
[mysqld2]
basedir = C:/mysql-4.0.17
port = 3308
enable-named-pipe
socket = mypipe2
```
Install the services as follows, using the full server pathnames to ensure that Windows registers the correct executable program for each service:
```
C:\> C:\mysql-4.0.8\bin\mysqld-nt --install mysqld1
C:\> C:\mysql-4.0.17\bin\mysqld-nt --install mysqld2
```
To start the services, use the services manager, or use NET START with the appropriate service names:
```
C:\> NET START mysqld1
C:\> NET START mysqld2
```
To stop the services, use the services manager, or use NET STOP with the appropriate service names:
```
C:\> NET STOP mysqld1
C:\> NET STOP mysqld2
```
Approach 2: Specify options for each server in separate files and use --defaults-file when you install the services to tell each server what file to use. In this case, each file should list options using a [mysqld] group.
With this approach, to specify options for the 4.0.8 mysqld-nt, create a file C:\my-opts1.cnf that looks like this:
```
[mysqld]
basedir = C:/mysql-4.0.8
port = 3307
enable-named-pipe
socket = mypipe1
```
For the 4.0.17 mysqld-nt, create a file C:\my-opts2.cnf that looks like this:
```
[mysqld]
basedir = C:/mysql-4.0.17
port = 3308
enable-named-pipe
socket = mypipe2
```
Install the services as follows (enter each command on a single line):
```
C:\> C:\mysql-4.0.8\bin\mysqld-nt --install mysqld1
           --defaults-file=C:\my-opts1.cnf
C:\> C:\mysql-4.0.17\bin\mysqld-nt --install mysqld2
           --defaults-file=C:\my-opts2.cnf
```
To use a --defaults-file option when you install a MySQL server as a service, you must precede the option with the service name.
After installing the services, start and stop them the same way as in the preceding example.

To remove multiple services, use mysqld --remove for each one, specifying a service name following the --remove option. If the service name is the default (MySQL), you can omit it.

5.7.2. Running Multiple Servers on Unix

The easiest way is to run multiple servers on Unix is to compile them with different TCP/IP ports and Unix socket files so that each one is listening on different network interfaces. Compiling in different base directories for each installation also results automatically in a separate, compiled-in data directory, log file, and PID file location for each server.

Assume that an existing server is configured for the default TCP/IP port number (3306) and Unix socket file (/tmp/mysql.sock). To configure a new server to have different operating parameters, use a configure command something like this:

shell> ./configure --with-tcp-port=port_number \
             --with-unix-socket-path=file_name \
             --prefix=/usr/local/mysql-4.0.17

Here, port_number and file_name must be different from the default TCP/IP port number and Unix socket file pathname, and the --prefix value should specify an installation directory different from the one under which the existing MySQL installation is located.

If you have a MySQL server listening on a given port number, you can use the following command to find out what operating parameters it is using for several important configurable variables, including the base directory and Unix socket filename:

shell> mysqladmin --host=host_name --port=port_number variables

With the information displayed by that command, you can tell what option values not to use when configuring an additional server.

Note that if you specify localhost as a hostname, mysqladmin defaults to using a Unix socket file connection rather than TCP/IP. In MySQL 4.1, you can explicitly specify the connection protocol to use by using the --protocol={TCP|SOCKET|PIPE|MEMORY} option.

You don't have to compile a new MySQL server just to start with a different Unix socket file and TCP/IP port number. It is also possible to use the same server binary and start each invocation of it with different parameter values at runtime. One way to do so is by using command-line options:

shell> mysqld_safe --socket=file_name --port=port_number

To start a second server, provide different --socket and --port option values, and pass a --datadir=path option to mysqld_safe so that the server uses a different data directory.

Another way to achieve a similar effect is to use environment variables to set the Unix socket filename and TCP/IP port number:

shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock
shell> MYSQL_TCP_PORT=3307
shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT
shell> mysql_install_db --user=mysql
shell> mysqld_safe --datadir=/path/to/datadir &

This is a quick way of starting a second server to use for testing. The nice thing about this method is that the environment variable settings apply to any client programs that you invoke from the same shell. Thus, connections for those clients are automatically directed to the second server.

Section 2.14, “Environment Variables”, includes a list of other environment variables you can use to affect mysqld.

For automatic server execution, the startup script that is executed at boot time should run the following command once for each server with an appropriate option file path for each command:

shell> mysqld_safe --defaults-file=file_name

Each option file should contain option values specific to a given server.

On Unix, the mysqld_multi script is another way to start multiple servers. See Section 4.3.4, “mysqld_multi — Manage Multiple MySQL Servers”.

5.7.3. Using Client Programs in a Multiple-Server Environment

To connect with a client program to a MySQL server that is listening to different network interfaces from those compiled into your client, you can use one of the following methods:

Start the client with --host=host_name --port=port_number to connect via TCP/IP to a remote server, with --host=127.0.0.1 --port=port_number to connect via TCP/IP to a local server, or with --host=localhost --socket=file_name to connect to a local server via a Unix socket file or a Windows named pipe.
As of MySQL 4.1, start the client with --protocol=TCP to connect via TCP/IP, --protocol=SOCKET to connect via a Unix socket file, --protocol=PIPE to connect via a named pipe, or --protocol=MEMORY to connect via shared memory. For TCP/IP connections, you may also need to specify --host and --port options. For the other types of connections, you may need to specify a --socket option to specify a Unix socket file or Windows named-pipe name, or a --shared-memory-base-name option to specify the shared-memory name. Shared-memory connections are supported only on Windows.
On Unix, set the MYSQL_UNIX_PORT and MYSQL_TCP_PORT environment variables to point to the Unix socket file and TCP/IP port number before you start your clients. If you normally use a specific socket file or port number, you can place commands to set these environment variables in your .login file so that they apply each time you log in. See Section 2.14, “Environment Variables”.
Specify the default Unix socket file and TCP/IP port number in the [client] group of an option file. For example, you can use C:\my.cnf on Windows, or the .my.cnf file in your home directory on Unix. See Section 4.2.3.2, “Using Option Files”.
In a C program, you can specify the socket file or port number arguments in the mysql_real_connect() call. You can also have the program read option files by calling mysql_options(). See Section 17.2.3, “C API Function Descriptions”.
If you are using the Perl DBD::mysql module, you can read options from MySQL option files. For example:
```
$dsn = "DBI:mysql:test;mysql_read_default_group=client;"
        . "mysql_read_default_file=/usr/local/mysql/data/my.cnf";
$dbh = DBI->connect($dsn, $user, $password);
```
See Section 17.4, “MySQL Perl API”.
Other programming interfaces may provide similar capabilities for reading option files.

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