Mysql：命令选项、配置选项、（全局、会话）系统变量、状态变量：系统变量

 

Server 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. 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: 

    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: 

    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 are present in all MySQL 5.1 releases. For historical information concerning their implementation, please see http://www.mysql.com/products/enterprise//5.0/en/, and http://www.mysql.com/products/enterprise//4.1/en/.

The following table lists all available system variables:

Table 5.2. mysqld System Variable Summary

Name Cmd-Line Option file System Var Var Scope Dynamic
auto_increment_increment Yes Yes Yes Both Yes
auto_increment_offset Yes Yes Yes Both Yes
autocommit     Yes Session Yes
automatic_sp_privileges     Yes Global Yes
back_log Yes Yes Yes Global No
basedir Yes Yes Yes Global No
big-tables Yes Yes     Yes
- Variable: big_tables     Yes Session Yes
binlog_cache_size Yes Yes Yes Global Yes
binlog-format Yes Yes     Yes
- Variable: binlog_format     Yes Both Yes
bulk_insert_buffer_size Yes Yes Yes Both Yes
character_set_client     Yes Both Yes
character_set_connection     Yes Both Yes
character_set_database[a]     Yes Both Yes
character-set-filesystem Yes Yes     Yes
- Variable: character_set_filesystem     Yes Both Yes
character_set_results     Yes Both Yes
character-set-server Yes Yes     Yes
- Variable: character_set_server     Yes Both Yes
character_set_system     Yes Global No
character-sets-dir Yes Yes     No
- Variable: character_sets_dir     Yes Global No
collation_connection     Yes Both Yes
collation_database[b]     Yes Both Yes
collation-server Yes Yes     Yes
- Variable: collation_server     Yes Both Yes
completion_type Yes Yes Yes Both Yes
concurrent_insert Yes Yes Yes Global Yes
connect_timeout Yes Yes Yes Global Yes
datadir Yes Yes Yes Global No
date_format     Yes Both Yes
datetime_format Yes Yes Yes Both Yes
debug Yes Yes Yes Both Yes
default_week_format Yes Yes Yes Both Yes
delay-key-write Yes Yes     Yes
- Variable: delay_key_write     Yes Global Yes
delayed_insert_limit Yes Yes Yes Global Yes
delayed_insert_timeout Yes Yes Yes Global Yes
delayed_queue_size Yes Yes Yes Global Yes
div_precision_increment Yes Yes Yes Both Yes
engine-condition-pushdown Yes Yes     Yes
- Variable: engine_condition_pushdown     Yes Both Yes
error_count     Yes Session No
event-scheduler Yes Yes     Yes
- Variable: event_scheduler     Yes Global Yes
expire_logs_days Yes Yes Yes Global Yes
flush Yes Yes Yes Global Yes
flush_time Yes Yes Yes Global Yes
foreign_key_checks     Yes Session Yes
ft_boolean_syntax Yes Yes Yes Global Yes
ft_max_word_len Yes Yes Yes Global No
ft_min_word_len Yes Yes Yes Global No
ft_query_expansion_limit Yes Yes Yes Global No
ft_stopword_file Yes Yes Yes Global No
general-log Yes Yes     Yes
- Variable: general_log     Yes Global Yes
general_log_file Yes Yes Yes Global Yes
group_concat_max_len Yes Yes Yes Both Yes
have_archive     Yes Global No
have_blackhole_engine     Yes Global No
have_compress     Yes Global No
have_crypt     Yes Global No
have_csv     Yes Global No
have_dynamic_loading     Yes Global No
have_example_engine     Yes Global No
have_federated_engine     Yes Global No
have_geometry     Yes Global No
have_innodb     Yes Global No
have_isam     Yes Global No
have_merge_engine     Yes Global No
have_ndbcluster     Yes Global No
have_openssl     Yes Global No
have_partitioning     Yes Global No
have_query_cache     Yes Global No
have_raid     Yes Global No
have_row_based_replication     Yes Global No
have_rtree_keys     Yes Global No
have_ssl     Yes Global No
have_symlink     Yes Global No
hostname     Yes Global No
identity     Yes Session Yes
ignore_builtin_innodb Yes Yes Yes Global No
init_connect Yes Yes Yes Global Yes
init-file Yes Yes     No
- Variable: init_file     Yes Global No
init_slave Yes Yes Yes Global Yes
innodb_adaptive_hash_index Yes Yes Yes Global No
innodb_additional_mem_pool_size Yes Yes Yes Global No
innodb_autoextend_increment Yes Yes Yes Global Yes
innodb_autoinc_lock_mode Yes Yes Yes Global No
innodb_buffer_pool_awe_mem_mb Yes Yes Yes Global No
innodb_buffer_pool_size Yes Yes Yes Global No
innodb_checksums Yes Yes Yes Global No
innodb_commit_concurrency Yes Yes Yes Global Yes
innodb_concurrency_tickets Yes Yes Yes Global Yes
innodb_data_file_path Yes Yes Yes Global No
innodb_data_home_dir Yes Yes Yes Global No
innodb_doublewrite Yes Yes Yes Global No
innodb_fast_shutdown Yes Yes Yes Global Yes
innodb_file_io_threads Yes Yes Yes Global No
innodb_file_per_table Yes Yes Yes Global No
innodb_flush_log_at_trx_commit Yes Yes Yes Global Yes
innodb_flush_method Yes Yes Yes Global No
innodb_force_recovery Yes Yes Yes Global No
innodb_lock_wait_timeout Yes Yes Yes Global No
innodb_locks_unsafe_for_binlog Yes Yes Yes Global No
innodb_log_arch_dir Yes Yes Yes Global No
innodb_log_archive Yes Yes Yes Global No
innodb_log_buffer_size Yes Yes Yes Global No
innodb_log_file_size Yes Yes Yes Global No
innodb_log_files_in_group Yes Yes Yes Global No
innodb_log_group_home_dir Yes Yes Yes Global No
innodb_max_dirty_pages_pct Yes Yes Yes Global Yes
innodb_max_purge_lag Yes Yes Yes Global Yes
innodb_mirrored_log_groups Yes Yes Yes Global No
innodb_open_files Yes Yes Yes Global No
innodb_rollback_on_timeout Yes Yes Yes Global No
innodb_stats_on_metadata Yes Yes Yes Global Yes
innodb_support_xa Yes Yes Yes Both Yes
innodb_sync_spin_loops Yes Yes Yes Global Yes
innodb_table_locks Yes Yes Yes Both Yes
innodb_thread_concurrency Yes Yes Yes Global Yes
innodb_thread_sleep_delay Yes Yes Yes Global Yes
innodb_use_legacy_cardinality_algorithm Yes Yes Yes Global Yes
insert_id     Yes Session Yes
interactive_timeout Yes Yes Yes Both Yes
join_buffer_size Yes Yes Yes Both Yes
keep_files_on_create Yes Yes Yes Both Yes
key_buffer_size Yes Yes Yes Global Yes
key_cache_age_threshold Yes Yes Yes Global Yes
key_cache_block_size Yes Yes Yes Global Yes
key_cache_division_limit Yes Yes Yes Global Yes
language Yes Yes Yes Global No
large_page_size     Yes Global No
large-pages Yes Yes     No
- Variable: large_pages     Yes Global No
last_insert_id     Yes Session Yes
lc_time_names     Yes Both Yes
license     Yes Global No
local_infile     Yes Global Yes
locked_in_memory     Yes Global No
log Yes Yes Yes Global Yes
log_bin     Yes Global No
log-bin Yes Yes Yes Global No
log-bin-trust-function-creators Yes Yes     Yes
- Variable: log_bin_trust_function_creators     Yes Global Yes
log-bin-trust-routine-creators Yes Yes     Yes
- Variable: log_bin_trust_routine_creators     Yes Global Yes
log-error Yes Yes     No
- Variable: log_error     Yes Global No
log-output Yes Yes     Yes
- Variable: log_output     Yes Global Yes
log-queries-not-using-indexes Yes Yes     Yes
- Variable: log_queries_not_using_indexes     Yes Global Yes
log-slave-updates Yes Yes     No
- Variable: log_slave_updates     Yes Global No
log-slow-queries Yes Yes     Yes
- Variable: log_slow_queries     Yes Global Yes
log-warnings Yes Yes     Yes
- Variable: log_warnings     Yes Both Yes
long_query_time Yes Yes Yes Both Yes
low-priority-updates Yes Yes     Yes
- Variable: low_priority_updates     Yes Both Yes
lower_case_file_system Yes Yes Yes Global No
lower_case_table_names Yes Yes Yes Global No
master-bind Yes Yes Yes   No
max_allowed_packet Yes Yes Yes Both Yes
max_binlog_cache_size Yes Yes Yes Global Yes
max_binlog_size Yes Yes Yes Global Yes
max_connect_errors Yes Yes Yes Global Yes
max_connections Yes Yes Yes Global Yes
max_delayed_threads Yes Yes Yes Both Yes
max_error_count Yes Yes Yes Both Yes
max_heap_table_size Yes Yes Yes Both Yes
max_insert_delayed_threads     Yes Both Yes
max_join_size Yes Yes Yes Both Yes
max_length_for_sort_data Yes Yes Yes Both Yes
max_prepared_stmt_count Yes Yes Yes Global Yes
max_relay_log_size Yes Yes Yes Global Yes
max_seeks_for_key Yes Yes Yes Both Yes
max_sort_length Yes Yes Yes Both Yes
max_sp_recursion_depth Yes Yes Yes Both Yes
max_tmp_tables Yes Yes Yes Both Yes
max_user_connections Yes Yes Yes Both Yes
max_write_lock_count Yes Yes Yes Global Yes
memlock Yes Yes Yes Global No
min-examined-row-limit Yes Yes Yes Both Yes
multi_range_count Yes Yes Yes Both Yes
myisam_data_pointer_size Yes Yes Yes Global Yes
myisam_max_sort_file_size Yes Yes Yes Global Yes
myisam_recover_options     Yes Global No
myisam_repair_threads Yes Yes Yes Both Yes
myisam_sort_buffer_size Yes Yes Yes Both Yes
myisam_stats_method Yes Yes Yes Both Yes
myisam_use_mmap Yes Yes Yes Global Yes
named_pipe     Yes Global No
ndb_autoincrement_prefetch_sz Yes Yes Yes Both Yes
ndb-batch-size Yes Yes Yes Global No
ndb_cache_check_time Yes Yes Yes Global Yes
ndb_extra_logging Yes Yes Yes Global Yes
ndb_force_send Yes Yes Yes Both Yes
ndb_log_empty_epochs Yes Yes Yes Global Yes
ndb_log_orig     Yes Global No
ndb-log-update-as-write Yes Yes Yes Global Yes
ndb_log_updated_only Yes Yes Yes Global Yes
ndb_optimization_delay     Yes Global Yes
ndb_table_no_logging     Yes Session Yes
ndb_table_temporary     Yes Session Yes
ndb_use_copying_alter_table     Yes Both No
ndb_use_exact_count     Yes Both Yes
ndb_wait_connected Yes Yes Yes   No
net_buffer_length Yes Yes Yes Both Yes
net_read_timeout Yes Yes Yes Both Yes
net_retry_count Yes Yes Yes Both Yes
net_write_timeout Yes Yes Yes Both Yes
new Yes Yes Yes Both Yes
old Yes Yes Yes Global No
old-passwords Yes Yes     Yes
- Variable: old_passwords     Yes Both Yes
open-files-limit Yes Yes     No
- Variable: open_files_limit     Yes Global No
optimizer_prune_level Yes Yes Yes Both Yes
optimizer_search_depth Yes Yes Yes Both Yes
optimizer_switch Yes Yes Yes Both Yes
pid-file Yes Yes     No
- Variable: pid_file     Yes Global No
plugin_dir Yes Yes Yes Global No
port Yes Yes Yes Global No
preload_buffer_size Yes Yes Yes Both Yes
prepared_stmt_count     Yes Global No
profiling     Yes Session Yes
profiling_history_size     Yes Both Yes
protocol_version     Yes Global No
query_alloc_block_size Yes Yes Yes Both Yes
query_cache_limit Yes Yes Yes Global Yes
query_cache_min_res_unit Yes Yes Yes Global Yes
query_cache_size Yes Yes Yes Global Yes
query_cache_type Yes Yes Yes Both Yes
query_cache_wlock_invalidate Yes Yes Yes Both Yes
query_prealloc_size Yes Yes Yes Both Yes
rand_seed1     Yes Session Yes
rand_seed2     Yes Session Yes
range_alloc_block_size Yes Yes Yes Both Yes
read_buffer_size Yes Yes Yes Both Yes
read_only Yes Yes Yes Global Yes
read_rnd_buffer_size Yes Yes Yes Both Yes
relay_log_purge Yes Yes Yes Global Yes
relay_log_space_limit Yes Yes Yes Global No
report-host Yes Yes     No
- Variable: report_host     Yes Global No
report-password Yes Yes     No
- Variable: report_password     Yes Global No
report-port Yes Yes     No
- Variable: report_port     Yes Global No
report-user Yes Yes     No
- Variable: report_user     Yes Global No
rpl_recovery_rank     Yes Global Yes
safe-show-database Yes Yes Yes Global Yes
secure-auth Yes Yes     Yes
- Variable: secure_auth     Yes Global Yes
secure-file-priv Yes Yes     No
- Variable: secure_file_priv     Yes Global No
server-id Yes Yes     Yes
- Variable: server_id     Yes Global Yes
shared_memory     Yes Global No
shared_memory_base_name     Yes Global No
skip-external-locking Yes Yes     No
- Variable: skip_external_locking     Yes Global No
skip-networking Yes Yes     No
- Variable: skip_networking     Yes Global No
skip-show-database Yes Yes     No
- Variable: skip_show_database     Yes Global No
slave-allow-batching Yes Yes     Yes
- Variable: slave_allow_batching     Yes Global Yes
slave_compressed_protocol Yes Yes Yes Global Yes
slave_exec_mode     Yes Global Yes
slave-load-tmpdir Yes Yes     No
- Variable: slave_load_tmpdir     Yes Global No
slave-net-timeout Yes Yes     Yes
- Variable: slave_net_timeout     Yes Global Yes
slave-skip-errors Yes Yes     No
- Variable: slave_skip_errors     Yes Global No
slave_transaction_retries Yes Yes Yes Global Yes
slow_launch_time Yes Yes Yes Global Yes
slow-query-log Yes Yes     Yes
- Variable: slow_query_log     Yes Global Yes
slow_query_log_file Yes Yes Yes Global Yes
socket Yes Yes Yes Global No
sort_buffer_size Yes Yes Yes Both Yes
sql_auto_is_null     Yes Session Yes
sql_big_selects     Yes Session Yes
sql_big_tables     Yes Session Yes
sql_buffer_result     Yes Session Yes
sql_log_bin     Yes Session Yes
sql_log_off     Yes Session Yes
sql_log_update     Yes Session Yes
sql_low_priority_updates     Yes Both Yes
sql_max_join_size     Yes Both Yes
sql-mode Yes Yes     Yes
- Variable: sql_mode     Yes Both Yes
sql_notes     Yes Session Yes
sql_quote_show_create     Yes Session Yes
sql_safe_updates     Yes Session Yes
sql_select_limit     Yes Both Yes
sql_slave_skip_counter     Yes Global Yes
sql_warnings     Yes Session Yes
ssl-ca Yes Yes     No
- Variable: ssl_ca     Yes Global No
ssl-capath Yes Yes     No
- Variable: ssl_capath     Yes Global No
ssl-cert Yes Yes     No
- Variable: ssl_cert     Yes Global No
ssl-cipher Yes Yes     No
- Variable: ssl_cipher     Yes Global No
ssl-key Yes Yes     No
- Variable: ssl_key     Yes Global No
storage_engine     Yes Both Yes
sync-binlog Yes Yes     Yes
- Variable: sync_binlog     Yes Global Yes
sync-frm Yes Yes     Yes
- Variable: sync_frm     Yes Global Yes
system_time_zone     Yes Global No
table_cache Yes Yes Yes Global Yes
table_definition_cache Yes Yes Yes Global Yes
table_lock_wait_timeout Yes Yes Yes Global Yes
table_open_cache Yes Yes Yes Global Yes
table_type     Yes Both Yes
thread_cache_size Yes Yes Yes Global Yes
thread_concurrency Yes Yes Yes Global No
thread_handling Yes Yes Yes Global No
thread_stack Yes Yes Yes Global No
time_format Yes Yes Yes Both Yes
time_zone Yes Yes Yes Both Yes
timed_mutexes Yes Yes Yes Global Yes
timestamp     Yes Session Yes
tmp_table_size Yes Yes Yes Both Yes
tmpdir Yes Yes Yes Global No
transaction_alloc_block_size Yes Yes Yes Both Yes
transaction_allow_batching     Yes Session Yes
transaction_prealloc_size Yes Yes Yes Both Yes
tx_isolation     Yes Both Yes
unique_checks     Yes Session Yes
updatable_views_with_limit Yes Yes Yes Both Yes
version Yes Yes Yes Global No
version_comment     Yes Global No
version_compile_machine     Yes Global No
version_compile_os     Yes Global No
wait_timeout Yes Yes Yes Both Yes
warning_count     Yes Session No

[a] This option is dynamic, but only the server should set this information. You should not set the value of this variable manually.

[b] This option is dynamic, but only the server should set this information. You should not set the value of this variable manually.

For additional system variable information, see these sections:

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.

Some system variables control the size of buffers or caches. For a given buffer, the server might need to allocate internal data structures. These structures typically are allocated from the total memory allocated to the buffer, and the amount of space required might be platform dependent. This means that when you assign a value to a system variable that controls a buffer size, the amount of space actually available might differ from the value assigned. In some cases, the amount might be less than the value assigned. It is also possible that the server will adjust a value upward. For example, if you assign a value of 0 to a variable for which the minimal value is 1024, the server will set the value to 1024.

  • automatic_sp_privileges

    Variable Name automatic_sp_privileges
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default TRUE

    When this variable has a value of 1 (the default), the server automatically grants the EXECUTE and ALTER ROUTINE privileges to the creator of a stored routine, if the user cannot already execute and alter or drop the routine. (The ALTER ROUTINE privilege is required to drop the routine.) The server also automatically drops those privileges when the creator drops the routine. If automatic_sp_privileges is 0, the server does not automatically add or drop these privileges.

  • back_log

    Command Line Format --back_log=#
    Config File Format back_log
    Option Sets Variable Yes, back_log
    Variable Name back_log
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type numeric
    Default 50
    Range 1-65535

    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

    Command Line Format --basedir=name
    Config File Format basedir
    Option Sets Variable Yes, basedir
    Variable Name basedir
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename

    The MySQL installation base directory. This variable can be set with the --basedir option. Relative path names for other variables usually are resolved relative to the base directory.

  • bulk_insert_buffer_size

    Command Line Format --bulk_insert_buffer_size=#
    Config File Format bulk_insert_buffer_size
    Option Sets Variable Yes, bulk_insert_buffer_size
    Variable Name bulk_insert_buffer_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 8388608
    Range 0-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 8388608
    Range 0-18446744073709547520

    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.

  • character_set_client

    Variable Name character_set_client
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type string

    The character set for statements that arrive from the client. 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

    Variable Name character_set_connection
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type string

    The character set used for literals that do not have a character set introducer and for number-to-string conversion.

  • character_set_database

    Variable Name character_set_database
    Variable Scope Both
    Dynamic Variable Yes
    Footnote This option is dynamic, but only the server should set this information. You should not set the value of this variable manually.
    Value Set
    Type string

    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.

  • character_set_filesystem

    Version Introduced 5.1.6
    Command Line Format --character-set-filesystem=name
    Config File Format character-set-filesystem
    Option Sets Variable Yes, character_set_filesystem
    Variable Name character_set_filesystem
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type string

    The file system character set. This variable is used to interpret string literals that refer to file names, such as in the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements and the LOAD_FILE() function. Such file names are converted from character_set_client to character_set_filesystem before the file opening attempt occurs. The default value is binary, which means that no conversion occurs. For systems on which multi-byte file names are allowed, a different value may be more appropriate. For example, if the system represents file names using UTF-8, set character_set_filesystem to 'utf8'. This variable was added in MySQL 5.1.6.

  • character_set_results

    Variable Name character_set_results
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type string

    The character set used for returning query results to the client.

  • character_set_server

    Command Line Format --character-set-server
    Config File Format character-set-server
    Option Sets Variable Yes, character_set_server
    Variable Name character_set_server
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type string

    The server's default character set.

  • character_set_system

    Variable Name character_set_system
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type string

    The character set used by the server for storing identifiers. The value is always utf8.

  • character_sets_dir

    Command Line Format --character-sets-dir=name
    Config File Format character-sets-dir
    Option Sets Variable Yes, character_sets_dir
    Variable Name character-sets-dir
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename

    The directory where character sets are installed.

  • collation_connection

    Variable Name collation_connection
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type string

    The collation of the connection character set.

  • collation_database

    Variable Name collation_database
    Variable Scope Both
    Dynamic Variable Yes
    Footnote This option is dynamic, but only the server should set this information. You should not set the value of this variable manually.
    Value Set
    Type string

    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.

  • collation_server

    Command Line Format --collation-server
    Config File Format collation-server
    Option Sets Variable Yes, collation_server
    Variable Name collation_server
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type string

    The server's default collation.

  • completion_type

    Command Line Format --completion_type=#
    Config File Format completion_type
    Option Sets Variable Yes, completion_type
    Variable Name competion_type
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 0
    Valid Values 0, 1, 2

    The transaction completion type:

    • If the value is 0 (the default), COMMIT and ROLLBACK are unaffected.

    • If the value is 1, COMMIT and ROLLBACK are equivalent to COMMIT AND CHAIN and ROLLBACK AND CHAIN, respectively. (A new transaction starts immediately with the same isolation level as the just-terminated transaction.)

    • If the value is 2, COMMIT and ROLLBACK are equivalent to COMMIT RELEASE and ROLLBACK RELEASE, respectively. (The server disconnects after terminating the transaction.)

  • concurrent_insert

    Command Line Format --concurrent_insert[=#]
    Config File Format concurrent_insert
    Option Sets Variable Yes, concurrent_insert
    Variable Name concurrent_insert
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 1
    Valid Values 0, 1, 2

    If 1 (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-mode or --skip-new.

    This variable can take three integer values.

    Value Description
    0 Off
    1 (Default) Enables concurrent insert for MyISAM tables that don't have holes
    2 Enables concurrent inserts for all MyISAM tables, even those that have holes. For a table with a hole, new rows are inserted at the end of the table if it is in use by another thread. Otherwise, MySQL acquires a normal write lock and inserts the row into the hole.

    See also Section 7.3.3, “Concurrent Inserts”.

  • connect_timeout

    Command Line Format --connect_timeout=#
    Config File Format connect_timeout
    Option Sets Variable Yes, connect_timeout
    Variable Name connect_timeout
    Variable Scope Global
    Dynamic Variable Yes
    Value Set (<= 5.1.22)
    Type numeric
    Default 5
    Value Set (>= 5.1.23)
    Type numeric
    Default 10

    The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake. The default value is 10 seconds as of MySQL 5.1.23 and 5 seconds before that.

    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.

  • datadir

    Command Line Format --datadir=name
    Config File Format datadir
    Option Sets Variable Yes, datadir
    Variable Name datadir
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename

    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.

  • debug

    Command Line Format --debug[=debug_options]
    Config File Format debug
    Variable Name debug
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type string
    Default 'd:t:o,/tmp/mysqld.trace

    This variable indicates the current debugging settings. It is available only for servers built with debugging support. The initial value comes from the value of instances of the --debug option given at server startup. The global and session values may be set at runtime; the SUPER privilege is required, even for the session value.

    Assigning a value that begins with + or - cause the value to added to or subtracted from the current value: 

    mysql> SET debug = 'T';
    mysql> SELECT @@debug;
    +---------+
    | @@debug |
    +---------+
    | T       | 
    +---------+

    mysql> SET debug = '+P';
    mysql> SELECT @@debug;
    +---------+
    | @@debug |
    +---------+
    | P:T     | 
    +---------+

    mysql> SET debug = '-P';
    mysql> SELECT @@debug;
    +---------+
    | @@debug |
    +---------+
    | T       | 
    +---------+

    This variable was added in MySQL 5.1.7.

  • default_week_format

    Command Line Format --default_week_format=#
    Config File Format default_week_format
    Option Sets Variable Yes, default_week_format
    Variable Name default_week_format
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 0
    Range 0-7

    The default mode value to use for the WEEK() function. See Section 11.6, “Date and Time Functions”.

  • delay_key_write

    Command Line Format --delay-key-write[=name]
    Config File Format delay-key-write
    Option Sets Variable Yes, delay_key_write
    Variable Name delay-key-write
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type enumeration
    Default ON
    Valid Values ON, OFF, ALL

    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, “Server Command Options”, and Section 13.5.1, “MyISAM Startup Options”.

    Warning

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

  • delayed_insert_limit

    Command Line Format --delayed_insert_limit=#
    Config File Format delayed_insert_limit
    Option Sets Variable Yes, delayed_insert_limit
    Variable Name delayed_insert_limit
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 100
    Range 1-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 100
    Range 1-18446744073709547520

    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

    Command Line Format --delayed_insert_timeout=#
    Config File Format delayed_insert_timeout
    Option Sets Variable Yes, delayed_insert_timeout
    Variable Name delayed_insert_timeout
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 300

    How many seconds an INSERT DELAYED handler thread should wait for INSERT statements before terminating.

  • delayed_queue_size

    Command Line Format --delayed_queue_size=#
    Config File Format delayed_queue_size
    Option Sets Variable Yes, delayed_queue_size
    Variable Name delayed_queue_size
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 1000
    Range 1-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 1000
    Range 1-18446744073709547520

    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.

  • div_precision_increment

    Command Line Format --div_precision_increment=#
    Config File Format div_precision_increment
    Option Sets Variable Yes, div_precision_increment
    Variable Name div_precision_increment
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 4
    Range 0-30

    This variable indicates the number of digits by which to increase the scale of the result of division operations performed with the / operator. The default value is 4. The minimum and maximum values are 0 and 30, respectively. The following example illustrates the effect of increasing the default value. 

    mysql> SELECT 1/7;
    +--------+
    | 1/7    |
    +--------+
    | 0.1429 |
    +--------+
    mysql> SET div_precision_increment = 12;
    mysql> SELECT 1/7;
    +----------------+
    | 1/7            |
    +----------------+
    | 0.142857142857 |
    +----------------+

  • engine_condition_pushdown

    Command Line Format --engine-condition-pushdown
    Config File Format engine-condition-pushdown
    Option Sets Variable Yes, engine_condition_pushdown
    Variable Name engine_condition_pushdown
    Variable Scope Both
    Dynamic Variable Yes
    Value Set (>= 5.1.0)
    Type boolean
    Default ON

    When the value of this variable is 0 (OFF), a query such as SELECT * FROM t WHERE mycol = 42, where mycol is a non-indexed column, is executed as a full table scan. The storage engine sends every row to the MySQL server, which applies the WHERE condition. If engine_condition_pushdown is set to 1 (ON), the condition is “pushed down” to the storage engine, which uses the condition to perform the scan, and sends back to the MySQL server only those rows that match the condition. By default, this variable is OFF.

    In MySQL 5.1, this variable is useful only with the NDBCLUSTER storage engine. However, we intend to implement it for additional storage engines in future MySQL releases.

    Setting this variable to ON on a MySQL Server acting as a MySQL Cluster SQL node causes WHERE conditions on unindexed columns to be evaluated on the cluster's data nodes and only the rows that match to be sent back to the SQL node that issued the query. This means the amount of cluster data that must be sent over the network is greatly reduced, increasing the efficiency with which results are returned.

    For more information, see Section 7.2.7, “Condition Pushdown Optimization”.

  • event_scheduler

    Version Introduced 5.1.6
    Command Line Format --event-scheduler[=value]
    Config File Format event-scheduler
    Option Sets Variable Yes, event_scheduler
    Variable Name event_scheduler
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type enumeration
    Default OFF
    Valid Values ON, OFF, DISABLED

    This variable indicates the status of the Event Scheduler; as of MySQL 5.1.12, possible values are ON, OFF, and DISABLED, with the default being OFF. This variable and its effects on the Event Scheduler's operation are discussed in greater detail in the Overview section of the Events chapter.

    This variable was added in MySQL 5.1.6.

  • expire_logs_days

    Command Line Format --expire_logs_days=#
    Config File Format expire_logs_days
    Option Sets Variable Yes, expire_logs_days
    Variable Name expire_logs_days
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 0
    Range 0-99

    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.

  • flush

    Command Line Format --flush
    Config File Format flush
    Variable Name flush
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default OFF

    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 B.1.4.2, “What to Do If MySQL Keeps Crashing”. This variable is set to ON if you start mysqld with the --flush option.

  • flush_time

    Command Line Format --flush_time=#
    Config File Format flush_time
    Option Sets Variable Yes, flush_time
    Variable Name flush_time
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 0
    Min Value 0
    Value Set
    Type (windows) numeric
    Default 1800
    Min Value 0

    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.

  • ft_boolean_syntax

    Command Line Format --ft_boolean_syntax=name
    Config File Format ft_boolean_syntax
    Variable Name ft_boolean_syntax
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type string
    Default +-><()~*:""&

    The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. See Section 11.8.2, “Boolean Full-Text Searches”.

    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

    Command Line Format --ft_max_word_len=#
    Config File Format ft_max_word_len
    Option Sets Variable Yes, ft_max_word_len
    Variable Name ft_max_word_len
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type numeric
    Min Value 10

    The maximum length of the word to be included in a FULLTEXT index.

    Note

    FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.

  • ft_min_word_len

    Command Line Format --ft_min_word_len=#
    Config File Format ft_min_word_len
    Option Sets Variable Yes, ft_min_word_len
    Variable Name ft_min_word_len
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type numeric
    Default 4
    Min Value 1

    The minimum length of the word to be included in a FULLTEXT index.

    Note

    FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.

  • ft_query_expansion_limit

    Command Line Format --ft_query_expansion_limit=#
    Config File Format ft_query_expansion_limit
    Option Sets Variable Yes, ft_query_expansion_limit
    Variable Name ft_query_expansion_limit
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type numeric
    Default 20
    Range 0-1000

    The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION.

  • ft_stopword_file

    Command Line Format --ft_stopword_file=name
    Config File Format ft_stopword_file
    Option Sets Variable Yes, ft_stopword_file
    Variable Name ft_stopword_file
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename

    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 storage/myisam/ft_static.c file). Setting this variable to the empty string ('') disables stopword filtering.

    Note

    FULLTEXT indexes must be rebuilt after changing this variable or the contents of the stopword file. Use REPAIR TABLE tbl_name QUICK.

  • general_log

    Version Introduced 5.1.12
    Command Line Format --general-log
    Config File Format general-log
    Option Sets Variable Yes, general_log
    Variable Name general_log
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default OFF

    Whether the general query log is enabled. The value can be 0 (or OFF) to disable the log or 1 (or ON) to enable the log. The default value depends on whether the --general_log option is given (--log before MySQL 5.1.29). The destination for log output is controlled by the log_output system variable; if that value is NONE, no log entries are written even if the log is enabled. The general_log variable was added in MySQL 5.1.12.

  • general_log_file

    Version Introduced 5.1.12
    Command Line Format
    --general-log-file=file_name 5.1.29
    Config File Format
    general_log_file 5.1.29
    Option Sets Variable Yes, general_log_file
    Variable Name general_log_file
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type filename
    Default host_name.log

    The name of the general query log file. The default value is host_name.log, but the initial value can be changed with the --general_log_file option (--log before MySQL 5.1.29). This variable was added in MySQL 5.1.12.

  • group_concat_max_len

    Command Line Format --group_concat_max_len=#
    Config File Format group_concat_max_len
    Option Sets Variable Yes, group_concat_max_len
    Variable Name group_concat_max_len
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 1024
    Range 4-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 1024
    Range 4-18446744073709547520

    The maximum allowed result length in bytes for the GROUP_CONCAT() function. The default is 1024.

  • have_archive

    YES if mysqld supports ARCHIVE tables, NO if not. This variable was removed in MySQL 5.1.14.

  • have_blackhole_engine

    YES if mysqld supports BLACKHOLE tables, NO if not. This variable was removed in MySQL 5.1.14.

  • 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.

  • have_crypt

    YES if the crypt() system call is available to the server, NO if not. If not, the ENCRYPT() function cannot be used.

  • have_csv

    YES if mysqld supports ARCHIVE tables, NO if not.

  • have_dynamic_loading

    YES if mysqld supports dynamic loading of plugins, NO if not. This variable was added in MySQL 5.1.10.

  • have_example_engine

    YES if mysqld supports EXAMPLE tables, NO if not. This variable was removed in MySQL 5.1.14.

  • have_federated_engine

    YES if mysqld supports FEDERATED tables, NO if not. This variable was removed in MySQL 5.1.14.

  • have_geometry

    YES if the server supports spatial data types, NO if not.

  • have_innodb

    YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used.

  • have_isam

    In MySQL 5.1, this variable appears only for reasons of backward compatibility. It is always NO because ISAM tables are no longer supported. This variable was removed in MySQL 5.1.7.

  • have_merge_engine

    YES if mysqld supports MERGE tables. DISABLED if --skip-merge is used. This variable was removed in MySQL 5.1.3.

  • have_openssl

    YES if mysqld supports SSL connections, NO if not. As of MySQL 5.1.17, this variable is an alias for have_ssl.

  • have_partitioning

    YES if mysqld supports partitioning. Added in MySQL 5.1.1 as have_partition_engine and renamed to have_partioning in 5.1.6.

  • have_query_cache

    YES if mysqld supports the query cache, NO if not.

  • have_row_based_replication

    Version Introduced 5.1.5
    Version Removed 5.1.15
    Variable Name have_row_based_replication
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type boolean

    YES if the server can perform replication using row-based binary logging. If the value is NO, the server can use only statement-based logging. See Section 16.1.2, “Replication Formats”. This variable was added in MySQL 5.1.5 and removed in 5.1.15.

  • have_raid

    In MySQL 5.1, this variable appears only for reasons of backward compatibility. It is always NO because RAID tables are no longer supported. This variable was removed in MySQL 5.1.7.

  • have_rtree_keys

    YES if RTREE indexes are available, NO if not. (These are used for spatial indexes in MyISAM tables.)

  • have_ssl

    YES if mysqld supports SSL connections, NO if not. This variable was added in MySQL 5.1.17. Before that, use have_openssl.

  • 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.

  • hostname

    Version Introduced 5.1.17
    Variable Name hostname
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type string

    The server sets this variable to the server host name at startup. This variable was added in MySQL 5.1.17.

  • init_connect

    Command Line Format --init-connect=name
    Config File Format init_connect
    Option Sets Variable Yes, init_connect
    Variable Name init_connect
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type string

    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.

  • init_file

    Command Line Format --init-file=name
    Config File Format init-file
    Option Sets Variable Yes, init_file
    Variable Name init_file
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename

    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.

    Note that the --init-file option is unavailable if MySQL was configured with the --disable-grant-options option. See Section 2.10.2, “Typical configure Options”.

  • innodb_xxx

    InnoDB system variables are listed in Section 13.6.3, “InnoDB Startup Options and System Variables”.

  • interactive_timeout

    Command Line Format --interactive_timeout=#
    Config File Format interactive_timeout
    Option Sets Variable Yes, interactive_timeout
    Variable Name interactive_timeout
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 28800
    Min Value 1

    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

    Command Line Format --join_buffer_size=#
    Config File Format join_buffer_size
    Option Sets Variable Yes, join_buffer_size
    Variable Name join_buffer_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 131072
    Range 8200-18446744073709547520

    The size of the buffer that is used for plain index scans, range index scans, and 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.

    The maximum allowable setting for join_buffer_size is 4GB. As of MySQL 5.1.23, values larger than 4GB are allowed for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB with a warning).

  • keep_files_on_create

    Version Introduced 5.1.21
    Command Line Format --keep_files_on_create=#
    Config File Format keep_files_on_create
    Option Sets Variable Yes, keep_files_on_create
    Variable Name keep_files_on_create
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default OFF

    If a MyISAM table is created with no DATA DIRECTORY option, the .MYD file is created in the database directory. By default, if MyISAM finds an existing .MYD file in this case, it overwrites it. The same applies to .MYI files for tables created with no INDEX DIRECTORY option. To suppress this behavior, set the keep_files_on_create variable to ON (1), in which case MyISAM will not overwrite existing files and returns an error instead. The default value is OFF (0).

    If a MyISAM table is created with a DATA DIRECTORY or INDEX DIRECTORY option and an existing .MYD or .MYI file is found, MyISAM always returns an error. It will not overwrite a file in the specified directory.

    This variable was added in MySQL 5.1.23.

  • key_buffer_size

    Command Line Format --key_buffer_size=#
    Config File Format key_buffer_size
    Option Sets Variable Yes, key_buffer_size
    Variable Name key_buffer_size
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 8388608
    Range 8-4294967295

    Index blocks for MyISAM 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 on 32-bit platforms. As of MySQL 5.1.23, values larger than 4GB are allowed for 64-bit platforms, except 64-bit Windows prior to MySQL 5.1.31, for which large values are truncated to 4GB with a warning. As of MySQL 5.1.31, values larger than 4GB are also allowed for 64-bit Windows. 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 file system caching for data reads, so you must leave some room for the file system 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 7.2.21, “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 12.5.5, “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, which is available from the key_cache_block_size system variable: 

    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.

    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 7.4.5, “The MyISAM Key Cache”.

  • key_cache_age_threshold

    Command Line Format --key_cache_age_threshold=#
    Config File Format key_cache_age_threshold
    Option Sets Variable Yes, key_cache_age_threshold
    Variable Name key_cache_age_threshold
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 300
    Range 100-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 300
    Range 100-18446744073709547520

    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. See Section 7.4.5, “The MyISAM Key Cache”.

  • key_cache_block_size

    Command Line Format --key_cache_block_size=#
    Config File Format key_cache_block_size
    Option Sets Variable Yes, key_cache_block_size
    Variable Name key_cache_block_size
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 1024
    Range 512-16384

    The size in bytes of blocks in the key cache. The default value is 1024. See Section 7.4.5, “The MyISAM Key Cache”.

  • key_cache_division_limit

    Command Line Format --key_cache_division_limit=#
    Config File Format key_cache_division_limit
    Option Sets Variable Yes, key_cache_division_limit
    Variable Name key_cache_division_limit
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 100
    Range 1-100

    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. See Section 7.4.5, “The MyISAM Key Cache”.

  • language

    Command Line Format --language=name
    Config File Format language
    Option Sets Variable Yes, language
    Variable Name language
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename
    Default /usr/local/mysql/share/mysql/english/

    The language used for error messages.

  • large_files_support

    Variable Name large_files_support
    Variable Scope Global
    Dynamic Variable No

    Whether mysqld was compiled with options for large file support.

  • large_pages

    Command Line Format --large-pages
    Config File Format large-pages
    Option Sets Variable Yes, large_pages
    Variable Name large_pages
    Variable Scope Global
    Dynamic Variable No
    Platform Specific linux
    Value Set
    Type (linux) boolean
    Default FALSE

    Whether large page support is enabled (via the --large-pages option). See Section 7.5.9, “Enabling Large Page Support”.

  • large_page_size

    Variable Name large_page_size
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type (linux) numeric
    Default 0

    If large page support is enabled, this shows the size of memory pages. Currently, large memory pages are supported only on Linux; on other platforms, the value of this variable is always 0. See Section 7.5.9, “Enabling Large Page Support”.

  • lc_time_names

    Version Introduced 5.1.12
    Variable Name lc_time_names
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type string

    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 5.1.12.

  • license

    Variable Name license
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type string
    Default GPL

    The type of license the server has.

  • local_infile

    Variable Name local_infile
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type

    Whether LOCAL is supported for LOAD DATA INFILE statements. See Section 5.3.4, “Security Issues with LOAD DATA LOCAL”.

  • locked_in_memory

    Variable Name locked_in_memory
    Variable Scope Global
    Dynamic Variable No

    Whether mysqld was locked in memory with --memlock.

  • log

    Whether logging of all statements to the general query log is enabled. See Section 5.2.3, “The General Query Log”.

    This variable is deprecated as of MySQL 5.1.29; use general_log instead.

  • log_bin

    Variable Name log_bin
    Variable Scope Global
    Dynamic Variable No

    Whether the binary log is enabled. See Section 5.2.4, “The Binary Log”.

  • log_bin_trust_function_creators

    Command Line Format --log-bin-trust-function-creators
    Config File Format log-bin-trust-function-creators
    Option Sets Variable Yes, log_bin_trust_function_creators
    Variable Name log_bin_trust_function_creators
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default FALSE

    This variable applies when binary logging is enabled. It controls whether stored function creators can be trusted not to create stored functions that will cause unsafe events to be written to the binary log. If set to 0 (the default), users are not allowed to create or alter stored functions unless they have the SUPER privilege in addition to the CREATE ROUTINE or ALTER ROUTINE privilege. A setting of 0 also enforces the restriction that a function must be declared with the DETERMINISTIC characteristic, or with the READS SQL DATA or NO SQL characteristic. If the variable is set to 1, MySQL does not enforce these restrictions on stored function creation. This variable also applies to trigger creation. See Section 19.6, “Binary Logging of Stored Programs”.

  • log_error

    Command Line Format --log-error[=name]
    Config File Format log-error
    Option Sets Variable Yes, log_error
    Variable Name log_error
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename

    The location of the error log.

  • log_output

    Version Introduced 5.1.6
    Command Line Format --log-output[=name]
    Config File Format log-output
    Option Sets Variable Yes, log_output
    Variable Name log_output
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type enumeration
    Default FILE
    Valid Values TABLE, FILE, NONE

    The destination for general query log and slow query log output. The value can be a comma-separated list of one or more of the words TABLE (log to tables), FILE (log to files), or NONE (do not log to tables or files). The default value is TABLE. NONE, if present, takes precedence over any other specifiers. If the value is NONE log entries are not written even if the logs are enabled. If the logs are not enabled, no logging occurs even if the value of log_output is not NONE. For more information, see Section 5.2.1, “Selecting General Query and Slow Query Log Output Destinations”. This variable was added in MySQL 5.1.6.

  • log_queries_not_using_indexes

    Version Deprecated 5.1.29
    Command Line Format --log-queries-not-using-indexes
    Config File Format log-queries-not-using-indexes
    Option Sets Variable Yes, log_queries_not_using_indexes
    Variable Name log_queries_not_using_indexes
    Variable Scope Global
    Dynamic Variable Yes
    Deprecated 5.1.29, by slow-query-log
    Value Set
    Type boolean

    Whether queries that do not use indexes are logged to the slow query log. See Section 5.2.5, “The Slow Query Log”. This variable was added in MySQL 5.1.11.

  • 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. See Section 16.1.3, “Replication and Binary Logging Options and Variables”.

  • log_slow_queries

    Command Line Format --log-slow-queries[=name]
    Config File Format log-slow-queries
    Option Sets Variable Yes, log_slow_queries
    Variable Name log_slow_queries
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type boolean

    Whether slow queries should be logged. “Slow” is determined by the value of the long_query_time variable. See Section 5.2.5, “The Slow Query Log”.

    This variable is deprecated as of MySQL 5.1.29; use slow_query_log instead.

  • log_warnings

    Command Line Format --log-warnings[=#]
    Config File Format log-warnings
    Option Sets Variable Yes, log_warnings
    Variable Name log_warnings
    Variable Scope Both
    Dynamic Variable Yes
    Disabled by skip-log-warnings
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 1
    Range 0-18446744073709547520

    Whether to produce additional warning messages. It is enabled (1) by default and can be disabled by setting it to 0. Aborted connections are not logged to the error log unless the value is greater than 1.

  • long_query_time

    Command Line Format --long_query_time=#
    Config File Format long_query_time
    Option Sets Variable Yes, long_query_time
    Variable Name long_query_time
    Variable Scope Both
    Dynamic Variable Yes
    Value Set (>= 5.1.21)
    Type numeric
    Default 10
    Min Value 0

    If a query takes longer than this many seconds, the server increments the Slow_queries status variable. If the slow query log is enabled, 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. Prior to MySQL 5.1.21, the minimum value is 1, and the value for this variable must be an integer. Beginning with MySQL 5.1.21, the minimum is 0, and a resolution of microseconds is supported when logging to a file. However, the microseconds part is ignored and only integer values are written when logging to tables. The default value is 10. See Section 5.2.5, “The Slow Query Log”.

  • low_priority_updates

    Command Line Format --low-priority-updates
    Config File Format low-priority-updates
    Option Sets Variable Yes, low_priority_updates
    Variable Name low_priority_updates
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default FALSE

    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). This variable previously was named sql_low_priority_updates.

  • lower_case_file_system

    Command Line Format --lower_case_file_system[=#]
    Config File Format lower_case_file_system
    Option Sets Variable Yes, lower_case_file_system
    Variable Name lower_case_file_system
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type boolean

    This variable describes the case sensitivity of file names on the file system where the data directory is located. OFF means file names are case sensitive, ON means they are not case sensitive.

  • lower_case_table_names

    Command Line Format --lower_case_table_names[=#]
    Config File Format lower_case_table_names
    Option Sets Variable Yes, lower_case_table_names
    Variable Name lower_case_table_names
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type numeric
    Default 0
    Range 0-2

    If set to 1, table names are stored in lowercase on disk and table name comparisons are not case sensitive. If set to 2 table names are stored as given but compared in lowercase. This option also applies to database names and table aliases. See Section 8.2.2, “Identifier Case Sensitivity”.

    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 file names (such as Windows or Mac OS X). If this variable is not set at startup and the file system on which the data directory is located does not have case-sensitive file names, MySQL automatically sets lower_case_table_names to 2.

  • max_allowed_packet

    Command Line Format --max_allowed_packet=#
    Config File Format max_allowed_packet
    Option Sets Variable Yes, max_allowed_packet
    Variable Name max_allowed_packet
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 1048576
    Range 1024-1073741824

    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 1GB. The value should be a multiple of 1024; non-multiples are rounded down to the nearest multiple.

    When you change the message buffer size by changing the value of the max_allowed_packet variable, you should also change the buffer size on the client side if your client program allows it. On the client side, max_allowed_packet has a default of 1GB. Some programs such as mysql and mysqldump enable you to change the client-side value by setting max_allowed_packet on the command line or in an option file.

    As of MySQL 5.1.31, the session value of this variable is read only. Before 5.1.31, setting the session value is allowed but has no effect.

  • max_connect_errors

    Command Line Format --max_connect_errors=#
    Config File Format max_connect_errors
    Option Sets Variable Yes, max_connect_errors
    Variable Name max_connect_errors
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 10
    Range 1-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 10
    Range 1-18446744073709547520

    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

    Command Line Format --max_connections=#
    Config File Format max_connections
    Option Sets Variable Yes, max_connections
    Variable Name max_connections
    Variable Scope Global
    Dynamic Variable Yes
    Value Set (<= 5.1.14)
    Type numeric
    Default 100
    Value Set (>= 5.1.15)
    Type numeric
    Default 151
    Range 1-16384
    Value Set (>= 5.1.17)
    Type numeric
    Default 151
    Range 1-100000

    The number of simultaneous client connections allowed. By default, this is 151, beginning with MySQL 5.1.15. (Previously, the default was 100.) See Section B.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 7.4.7, “How MySQL Opens and Closes Tables”, for comments on file descriptor limits.

  • max_delayed_threads

    Command Line Format --max_delayed_threads=#
    Config File Format max_delayed_threads
    Option Sets Variable Yes, max_delayed_threads
    Variable Name max_delayed_threads
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 20
    Range 0-16384

    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, this disables DELAYED entirely.

    For the SESSION value of this variable, the only valid values are 0 or the GLOBAL value.

  • max_error_count

    Command Line Format --max_error_count=#
    Config File Format max_error_count
    Option Sets Variable Yes, max_error_count
    Variable Name max_error_count
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 64
    Range 0-65535

    The maximum number of error, warning, and note messages to be stored for display by the SHOW ERRORS and SHOW WARNINGS statements.

  • max_heap_table_size

    Command Line Format --max_heap_table_size=#
    Config File Format max_heap_table_size
    Option Sets Variable Yes, max_heap_table_size
    Variable Name max_heap_table_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 16777216
    Range 16384-4294967295

    This variable sets the maximum size to which MEMORY 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.

    Note

    On 64-bit platforms, the maximum value for this variable is 1844674407370954752.

    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

    Variable Name max_insert_delayed_threads
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric

    This variable is a synonym for max_delayed_threads.

  • max_join_size

    Command Line Format --max_join_size=#
    Config File Format max_join_size
    Option Sets Variable Yes, max_join_size
    Variable Name max_join_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 4294967295
    Range 1-4294967295

    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

    Command Line Format --max_length_for_sort_data=#
    Config File Format max_length_for_sort_data
    Option Sets Variable Yes, max_length_for_sort_data
    Variable Name max_length_for_sort_data
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 1024
    Range 4-8388608

    The cutoff on the size of index values that determines which filesort algorithm to use. See Section 7.2.13, “ORDER BY Optimization”.

  • max_prepared_stmt_count

    Version Introduced 5.1.10
    Command Line Format
    --max_prepared_stmt_count=# 5.0.21
    Config File Format
    max_prepared_stmt_count 5.0.21
    Option Sets Variable Yes, max_prepared_stmt_count
    Variable Name max_prepared_stmt_count
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 16382
    Range 0-1048576

    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. 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. The default value is 16,382. The allowable range of values is from 0 to 1 million. Setting the value to 0 disables prepared statements. This variable was added in MySQL 5.1.10.

  • max_relay_log_size

    Command Line Format --max_relay_log_size=#
    Config File Format max_relay_log_size
    Option Sets Variable Yes, max_relay_log_size
    Variable Name max_relay_log_size
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 0
    Range 0-1073741824

    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. See Section 16.4.1, “Replication Implementation Details”.

  • max_seeks_for_key

    Command Line Format --max_seeks_for_key=#
    Config File Format max_seeks_for_key
    Option Sets Variable Yes, max_seeks_for_key
    Variable Name max_seeks_for_key
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 4294967295
    Range 1-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 18446744073709547520
    Range 1-18446744073709547520

    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 12.5.5.23, “SHOW INDEX Syntax”). By setting this to a low value (say, 100), you can force MySQL to prefer indexes instead of table scans.

  • max_sort_length

    Command Line Format --max_sort_length=#
    Config File Format max_sort_length
    Option Sets Variable Yes, max_sort_length
    Variable Name max_sort_length
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 1024
    Range 4-8388608

    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_sp_recursion_depth

    Command Line Format --max_sp_recursion_depth[=#]
    Config File Format max_sp_recursion_depth
    Option Sets Variable Yes, max_sp_recursion_depth
    Variable Name max_sp_recursion_depth
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 0
    Max Value 255

    The number of times that any given stored procedure may be called recursively. The default value for this option is 0, which completely disallows recursion in stored procedures. The maximum value is 255.

    Stored procedure recursion increases the demand on thread stack space. If you increase the value of max_sp_recursion_depth, it may be necessary to increase thread stack size by increasing the value of thread_stack at server startup.

  • max_tmp_tables

    Command Line Format --max_tmp_tables=#
    Config File Format max_tmp_tables
    Option Sets Variable Yes, max_tmp_tables
    Variable Name max_tmp_tables
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 32
    Range 1-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 32
    Range 1-18446744073709547520

    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

    Command Line Format --max_user_connections=#
    Config File Format max_user_connections
    Option Sets Variable Yes, max_user_connections
    Variable Name max_user_connections
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Range 1-4294967295

    The maximum number of simultaneous connections allowed to any given MySQL account. A value of 0 means “no limit.”

    This variable has both a global scope and a (read-only) session scope. The session variable has the same value as the global variable unless the current account has a non-zero MAX_USER_CONNECTIONS resource limit. In that case, the session value reflects the account limit.

  • max_write_lock_count

    Command Line Format --max_write_lock_count=#
    Config File Format max_write_lock_count
    Option Sets Variable Yes, max_write_lock_count
    Variable Name max_write_lock_count
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 4294967295
    Range 1-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 18446744073709547520
    Range 1-18446744073709547520

    After this many write locks, allow some pending read lock requests to be processed in between.

  • min_examined_row_limit

    Version Introduced 5.1.21
    Command Line Format --min-examined-row-limit=#
    Config File Format min-examined-row-limit
    Variable Name min_examined_row_limit
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 0
    Range 0-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 0
    Range 0-18446744073709547520

    Queries that examine fewer than this number of rows are not logged to the slow query log. This variable was added in MySQL 5.1.21.

  • myisam_data_pointer_size

    Command Line Format --myisam_data_pointer_size=#
    Config File Format myisam_data_pointer_size
    Option Sets Variable Yes, myisam_data_pointer_size
    Variable Name myisam_data_pointer_size
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 6
    Range 2-7

    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 6. See Section B.1.2.12, “The table is full”.

  • myisam_max_sort_file_size

    Command Line Format --myisam_max_sort_file_size=#
    Config File Format myisam_max_sort_file_size
    Option Sets Variable Yes, myisam_max_sort_file_size
    Variable Name myisam_max_sort_file_size
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 2147483648

    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. The value is given in bytes.

    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

    Variable Name myisam_recover_options
    Variable Scope Global
    Dynamic Variable No

    The value of the --myisam-recover option. See Section 5.1.2, “Server Command Options”.

  • myisam_repair_threads

    Command Line Format --myisam_repair_threads=#
    Config File Format myisam_repair_threads
    Option Sets Variable Yes, myisam_repair_threads
    Variable Name myisam_repair_threads
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 1
    Range 1-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 1
    Range 1-18446744073709547520

    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.

  • myisam_sort_buffer_size

    Command Line Format --myisam_sort_buffer_size=#
    Config File Format myisam_sort_buffer_size
    Option Sets Variable Yes, myisam_sort_buffer_size
    Variable Name myisam_sort_buffer_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 8388608
    Range 4-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 8388608
    Range 4-18446744073709547520

    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.

    The maximum allowable setting for myisam_sort_buffer_size is 4GB. As of MySQL 5.1.23, values larger than 4GB are allowed for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB with a warning).

  • myisam_stats_method

    Command Line Format --myisam_stats_method=name
    Config File Format myisam_stats_method
    Option Sets Variable Yes, myisam_stats_method
    Variable Name myisam_stats_method
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type enumeration
    Valid Values nulls_equal, nulls_unequal, nulls_ignored

    How the server treats NULL values when collecting statistics about the distribution of index values for MyISAM tables. This variable has three possible values, nulls_equal, nulls_unequal, and nulls_ignored. 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. For nulls_ignored, NULL values are ignored.

    The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 7.4.6, “MyISAM Index Statistics Collection”.

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

  • myisam_use_mmap

    Version Introduced 5.1.4
    Command Line Format --myisam_use_mmap
    Config File Format myisam_use_mmap
    Option Sets Variable Yes, myisam_use_mmap
    Variable Name myisam_use_mmap
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default FALSE

    Use memory mapping for reading and writing MyISAM tables. This variable was added in MySQL 5.1.4.

  • named_pipe

    Variable Name named_pipe
    Variable Scope Global
    Dynamic Variable No
    Platform Specific windows

    (Windows only.) Indicates whether the server supports connections over named pipes.

  • net_buffer_length

    Command Line Format --net_buffer_length=#
    Config File Format net_buffer_length
    Option Sets Variable Yes, net_buffer_length
    Variable Name net_buffer_length
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 16384
    Range 1024-1048576

    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.

    As of MySQL 5.1.31, the session value of this variable is read only. Before 5.1.31, setting the session value is allowed but has no effect.

  • net_read_timeout

    Command Line Format --net_read_timeout=#
    Config File Format net_read_timeout
    Option Sets Variable Yes, net_read_timeout
    Variable Name net_read_timeout
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 30
    Min Value 1

    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.

  • net_retry_count

    Command Line Format --net_retry_count=#
    Config File Format net_retry_count
    Option Sets Variable Yes, net_retry_count
    Variable Name net_retry_count
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 10
    Range 1-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 10
    Range 1-18446744073709547520

    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.

  • net_write_timeout

    Command Line Format --net_write_timeout=#
    Config File Format net_write_timeout
    Option Sets Variable Yes, net_write_timeout
    Variable Name net_write_timeout
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 60
    Min Value 1

    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.

  • new

    Command Line Format --new
    Config File Format new
    Option Sets Variable Yes, new
    Variable Name new
    Variable Scope Both
    Dynamic Variable Yes
    Disabled by skip-new
    Value Set
    Type boolean
    Default FALSE

    This variable was used in MySQL 4.0 to turn on some 4.1 behaviors, and is retained for backward compatibility. In MySQL 5.1, its value is always OFF.

  • old

    Version Introduced 5.1.18
    Command Line Format old
    Config File Format old
    Variable Name old
    Variable Scope Global
    Dynamic Variable No

    old is a compatibility variable. It is disabled by default, but can be enabled at startup to revert the server to behaviors present in older versions.

    Currently, when old is enabled, it changes the default scope of index hints to that used prior to MySQL 5.1.17. That is, index hints with no FOR clause apply only to how indexes are used for row retrieval and not to resolution of ORDER BY or GROUP BY clauses. (See Section 12.2.8.2, “Index Hint Syntax”.) Take care about enabling this in a replication setup. With statement-based binary logging, having different modes for the master and slaves might lead to replication errors.

    This variable was added as old_mode in MySQL 5.1.17 and renamed to old in MySQL 5.1.18.

  • old_passwords

    Command Line Format --old_passwords
    Config File Format old-passwords
    Option Sets Variable Yes, old_passwords
    Variable Name old_passwords
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default FALSE

    Whether the server should use pre-4.1-style passwords for MySQL user accounts. See Section B.1.2.4, “Client does not support authentication protocol”.

  • one_shot

    This is not a variable, but it can be used when setting some variables. It is described in Section 12.5.4, “SET Syntax”.

  • open_files_limit

    Command Line Format --open-files-limit=#
    Config File Format open-files-limit
    Option Sets Variable Yes, open_files_limit
    Variable Name open_files_limit
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type numeric
    Default 0
    Range 0-65535

    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.

  • optimizer_prune_level

    Command Line Format --optimizer_prune_level[=#]
    Config File Format optimizer_prune_level
    Option Sets Variable Yes, optimizer_prune_level
    Variable Name optimizer_prune_level
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default 1

    Controls the heuristics applied during query optimization to prune less-promising partial plans from the optimizer search space. A value of 0 disables heuristics so that the optimizer performs an exhaustive search. A value of 1 causes the optimizer to prune plans based on the number of rows retrieved by intermediate plans.

  • optimizer_search_depth

    Command Line Format --optimizer_search_depth[=#]
    Config File Format optimizer_search_depth
    Option Sets Variable Yes, optimizer_search_depth
    Variable Name optimizer_search_depth
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 62

    The maximum depth of search performed by the query optimizer. Values larger than the number of relations in a query result in better query plans, but take longer to generate an execution plan for a query. Values smaller than the number of relations in a query return an execution plan quicker, but the resulting plan may be far from being optimal. If set to 0, the system automatically picks a reasonable value. If set to the maximum number of tables used in a query plus 2, the optimizer switches to the algorithm used in MySQL 5.0.0 (and previous versions) for performing searches.

  • optimizer_switch

    Version Introduced 5.1.34
    Command Line Format --optimizer_switch=value
    Config File Format optimizer_switch
    Option Sets Variable Yes, optimizer_switch
    Variable Name optimizer_switch
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type set
    Valid Values index_merge={on|off}, index_merge_intersection={on|off}, index_merge_union={on|off}, index_merge_sort_union={on|off}

    The optimizer_switch system variable enables control over optimizer behaviors. The value of this variable is a set of flags, each of which has a value of on or off to indicate whether the corresponding optimizer behavior is enabled or disabled. This variable has global and session values and be changed at runtime. The global default can be set at server startup.

    To see the current set of optimizer flags, select the variable value: 

    mysql> SELECT @@optimizer_switch\G
    *************************** 1. row ***************************
    @@optimizer_switch: index_merge=on,index_merge_union=on,
                        index_merge_sort_union=on,index_merge_intersection=on

    For more information about the syntax of this variable and the optimizer behaviors that it controls, see Section 7.2.18, “Using optimizer_switch to Control the Optimizer”.

  • pid_file

    Command Line Format --pid-file=name
    Config File Format pid-file
    Option Sets Variable Yes, pid_file
    Variable Name pid_file
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename

    The path name of the process ID (PID) file. This variable can be set with the --pid-file option.

  • plugin_dir

    Version Introduced 5.1.2
    Command Line Format --plugin_dir=name
    Config File Format plugin_dir
    Option Sets Variable Yes, plugin_dir
    Variable Name plugin_dir
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename
    Default /usr/local/mysql/lib/mysql

    The path name of the plugin directory. This variable was added in MySQL 5.1.2.

  • port

    Command Line Format --port=#
    Config File Format port
    Option Sets Variable Yes, port
    Variable Name port
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type numeric
    Default 3306

    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

    Command Line Format --preload_buffer_size=#
    Config File Format preload_buffer_size
    Option Sets Variable Yes, preload_buffer_size
    Variable Name preload_buffer_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 32768
    Range 1024-1073741824

    The size of the buffer that is allocated when preloading indexes.

  • 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 5.1.10. In MySQL 5.1.14, it was converted to the global Prepared_stmt_count status variable.

  • protocol_version

    Variable Name protocol_version
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type numeric

    The version of the client/server protocol used by the MySQL server.

  • query_alloc_block_size

    Command Line Format --query_alloc_block_size=#
    Config File Format query_alloc_block_size
    Option Sets Variable Yes, query_alloc_block_size
    Variable Name query_alloc_block_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 8192
    Range 1024-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 8192
    Range 1024-18446744073709547520

    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.

  • query_cache_limit

    Command Line Format --query_cache_limit=#
    Config File Format query_cache_limit
    Option Sets Variable Yes, query_cache_limit
    Variable Name query_cache_limit
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 1048576
    Range 0-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 1048576
    Range 0-18446744073709547520

    Don't cache results that are larger than this number of bytes. The default value is 1MB.

  • query_cache_min_res_unit

    Command Line Format --query_cache_min_res_unit=#
    Config File Format query_cache_min_res_unit
    Option Sets Variable Yes, query_cache_min_res_unit
    Variable Name query_cache_min_res_unit
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 4096
    Range 512-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 4096
    Range 512-18446744073709547520

    The minimum size (in bytes) for blocks allocated by the query cache. The default value is 4096 (4KB). Tuning information for this variable is given in Section 7.5.5.3, “Query Cache Configuration”.

  • query_cache_size

    Command Line Format --query_cache_size=#
    Config File Format query_cache_size
    Option Sets Variable Yes, query_cache_size
    Variable Name query_cache_size
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 0
    Range 0-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 0
    Range 0-18446744073709547520

    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. See Section 7.5.5.3, “Query Cache Configuration”, for more information.

    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 7.5.5.3, “Query Cache Configuration”.

  • query_cache_type

    Command Line Format --query_cache_type=#
    Config File Format query_cache_type
    Option Sets Variable Yes, query_cache_type
    Variable Name query_cache_type
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type enumeration
    Default 1
    Valid Values 0, 1, 2

    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. Possible values are shown in the following table.

    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.

  • query_cache_wlock_invalidate

    Command Line Format --query_cache_wlock_invalidate
    Config File Format query_cache_wlock_invalidate
    Option Sets Variable Yes, query_cache_wlock_invalidate
    Variable Name query_cache_wlock_invalidate
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default FALSE

    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.

  • query_prealloc_size

    Command Line Format --query_prealloc_size=#
    Config File Format query_prealloc_size
    Option Sets Variable Yes, query_prealloc_size
    Variable Name query_prealloc_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 8192
    Range 8192-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 8192
    Range 8192-18446744073709547520

    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.

  • range_alloc_block_size

    Command Line Format --range_alloc_block_size=#
    Config File Format range_alloc_block_size
    Option Sets Variable Yes, range_alloc_block_size
    Variable Name range_alloc_block_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 4096
    Range 4096-18446744073709547520

    The size of blocks that are allocated when doing range optimization.

  • read_buffer_size

    Command Line Format --read_buffer_size=#
    Config File Format read_buffer_size
    Option Sets Variable Yes, read_buffer_size
    Variable Name read_buffer_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 131072
    Range 8200-2147479552

    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, which defaults to 131072. The value of this variable should be a multiple of 4KB. If it is set to a value that is not a multiple of 4KB, its value will be rounded down to the nearest multiple of 4KB.

    The maximum allowable setting for read_buffer_size is 2GB.

    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 7.5.8, “How MySQL Uses Memory”, for example.

  • read_only

    Command Line Format --read_only
    Config File Format read_only
    Option Sets Variable Yes, read_only
    Variable Name read_only
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 0

    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. This variable does not apply to TEMPORARY tables, nor does it prevent the server from inserting rows into the log tables (see Section 5.2.1, “Selecting General Query and Slow Query Log Output Destinations”).

    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.

    As of MySQL 5.1.15, the following conditions apply:

    • If you attempt to enable read_only while you have any explicit locks (acquired with LOCK TABLES) or have a pending transaction, an error occurs.

    • If you attempt to enable read_only while other clients hold explicit table locks or have pending transactions, the attempt blocks until the locks are released and the transactions end. While the attempt to enable read_only is pending, requests by other clients for table locks or to begin transactions also block until read_only has been set.

    • read_only can be enabled while you hold a global read lock (acquired with FLUSH TABLES WITH READ LOCK) because that does not involve table locks.

  • read_rnd_buffer_size

    Command Line Format --read_rnd_buffer_size=#
    Config File Format read_rnd_buffer_size
    Option Sets Variable Yes, read_rnd_buffer_size
    Variable Name read_rnd_buffer_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 262144
    Range 8200-4294967295

    When reading rows in sorted order following a key-sorting operation, the rows are read through this buffer to avoid disk seeks. See Section 7.2.13, “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.

    The maximum allowable setting for read_rnd_buffer_size is 2GB.

    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 7.5.8, “How MySQL Uses Memory”, for example.

  • relay_log_purge

    Command Line Format --relay_log_purge
    Config File Format relay_log_purge
    Option Sets Variable Yes, relay_log_purge
    Variable Name relay_log_purge
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default TRUE

    Disables or enables automatic purging of relay log files as soon as they are not needed any more. The default value is 1 (ON).

  • relay_log_space_limit

    Command Line Format --relay_log_space_limit=#
    Config File Format 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
    Platform Bit Size 32
    Type numeric
    Default 0
    Range 0-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 0
    Range 0-18446744073709547520

    The maximum amount of space to use for all relay logs.

  • report_host

    The value of the --report-host option. This variable was added in MySQL 5.1.24.

  • report_password

    The value of the --report-password option. This variable was added in MySQL 5.1.24.

  • report_port

    The value of the --report-port option. This variable was added in MySQL 5.1.24.

  • report_user

    The value of the --report-user option. This variable was added in MySQL 5.1.24.

  • secure_auth

    Command Line Format --secure-auth
    Config File Format secure-auth
    Option Sets Variable Yes, secure_auth
    Variable Name secure_auth
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default FALSE

    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 employing the old format (and hence insecure communication over the network).

    Server startup fails with an error if this option is enabled and the privilege tables are in pre-4.1 format. See Section B.1.2.4, “Client does not support authentication protocol”.

  • secure_file_priv

    Version Introduced 5.1.17
    Command Line Format --secure-file-priv
    Config File Format secure-file-priv
    Option Sets Variable Yes, secure_file_priv
    Variable Name secure_file_priv
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type string

    By default, this variable is empty. If set to the name of a directory, it limits the effect of the LOAD_FILE() function and the LOAD DATA and SELECT ... INTO OUTFILE statements to work only with files in that directory.

    This variable was added in MySQL 5.1.17.

  • server_id

    Command Line Format --server-id=#
    Config File Format server-id
    Option Sets Variable Yes, server_id
    Variable Name server_id
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 0
    Range 0-4294967295

    The server ID, used in replication to give each master and slave a unique identity. This variable is set by the --server-id option. For each server participating in replication, you should pick a positive integer in the range from 1 to 232 – 1 to act as that server's ID.

  • shared_memory

    Variable Name shared_memory
    Variable Scope Global
    Dynamic Variable No
    Platform Specific windows

    (Windows only.) Whether the server allows shared-memory connections.

  • shared_memory_base_name

    Variable Name shared_memory_base_name
    Variable Scope Global
    Dynamic Variable No
    Platform Specific windows

    (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. The default name is MYSQL. The name is case sensitive.

  • skip_external_locking

    This is OFF if mysqld uses external locking, ON if external locking is disabled.

  • 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.

  • 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 have concerns about users being able to see databases belonging to other users. Its effect 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 the names of only those databases for which the user has the SHOW DATABASES or other privilege.

  • slow_launch_time

    Command Line Format --slow_launch_time=#
    Config File Format slow_launch_time
    Option Sets Variable Yes, slow_launch_time
    Variable Name slow_launch_time
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 2

    If creating a thread takes longer than this many seconds, the server increments the Slow_launch_threads status variable.

  • slow_query_log

    Whether the slow query log is enabled. The value can be 0 (or OFF) to disable the log or 1 (or ON) to enable the log. The default value depends on whether the --slow_query_log option is given (--log-slow-queries before MySQL 5.1.29). The destination for log output is controlled by the log_output system variable; if that value is NONE, no log entries are written even if the log is enabled. The slow_query_log variable was added in MySQL 5.1.12.

  • slow_query_log_file

    Version Introduced 5.1.12
    Command Line Format --slow-query-log-file=file_name
    Config File Format slow_query_log_file
    Option Sets Variable Yes, slow_query_log_file
    Variable Name slow_query_log_file
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type filename

    The name of the slow query log file. The default value is host_name-slow.log, but the initial value can be changed with the --slow_query_log_file option (--log-slow-queries before MySQL 5.1.29). This variable was added in MySQL 5.1.12.

  • socket

    Command Line Format --socket=name
    Config File Format socket
    Option Sets Variable Yes, socket
    Variable Name socket
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename
    Default /tmp/mysql.sock

    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

    Command Line Format --sort_buffer_size=#
    Config File Format sort_buffer_size
    Option Sets Variable Yes, sort_buffer_size
    Variable Name sort_buffer_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 2097144
    Max Value 4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 2097144
    Max Value 18446744073709547520

    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 B.1.4.4, “Where MySQL Stores Temporary Files”.

    The maximum allowable setting for sort_buffer_size is 4GB. As of MySQL 5.1.23, values larger than 4GB are allowed for 64-bit platforms (except 64-bit Windows, for which large values are truncated to 4GB with a warning).

  • sql_mode

    Command Line Format --sql-mode=name
    Config File Format sql-mode
    Option Sets Variable Yes, sql_mode
    Variable Name sql_mode
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type set
    Default ''
    Valid Values ALLOW_INVALID_DATES, ANSI_QUOTES, ERROR_FOR_DIVISION_BY_ZERO, HIGH_NOT_PRECEDENCE, IGNORE_SPACE, NO_AUTO_CREATE_USER, NO_AUTO_VALUE_ON_ZERO, NO_BACKSLASH_ESCAPES, NO_DIR_IN_CREATE, NO_ENGINE_SUBSTITUTION, NO_FIELD_OPTIONS, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_UNSIGNED_SUBTRACTION, NO_ZERO_DATE, NO_ZERO_IN_DATE, ONLY_FULL_GROUP_BY, PAD_CHAR_TO_FULL_LENGTH, PIPES_AS_CONCAT, REAL_AS_FLOAT, STRICT_ALL_TABLES, STRICT_TRANS_TABLES

    The current server SQL mode, which can be set dynamically. See Section 5.1.7, “Server SQL Modes”.

  • sql_select_limit

    Variable Name sql_select_limit
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric

    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.

  • ssl_ca

    Version Introduced 5.1.11
    Command Line Format --ssl-ca=name
    Config File Format ssl-ca
    Option Sets Variable Yes, ssl_ca
    Variable Name ssl_ca
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename

    The path to a file with a list of trusted SSL CAs. This variable was added in MySQL 5.1.11.

  • ssl_capath

    Version Introduced 5.1.11
    Command Line Format --ssl-capath=name
    Config File Format ssl-capath
    Option Sets Variable Yes, ssl_capath
    Variable Name ssl_capath
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename

    The path to a directory that contains trusted SSL CA certificates in PEM format. This variable was added in MySQL 5.1.11.

  • ssl_cert

    Version Introduced 5.1.11
    Command Line Format --ssl-cert=name
    Config File Format ssl-cert
    Option Sets Variable Yes, ssl_cert
    Variable Name ssl_cert
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename

    The name of the SSL certificate file to use for establishing a secure connection. This variable was added in MySQL 5.1.11.

  • ssl_cipher

    Version Introduced 5.1.11
    Command Line Format --ssl-cipher=name
    Config File Format ssl-cipher
    Option Sets Variable Yes, ssl_cipher
    Variable Name ssl_cipher
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename

    A list of allowable ciphers to use for SSL encryption. This variable was added in MySQL 5.1.11.

  • ssl_key

    Version Introduced 5.1.11
    Command Line Format --ssl-key=name
    Config File Format ssl-key
    Option Sets Variable Yes, ssl_key
    Variable Name ssl_key
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type string

    The name of the SSL key file to use for establishing a secure connection. This variable was added in MySQL 5.1.11.

  • storage_engine

    Variable Name storage_engine
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type enumeration

    The default storage engine (table type). To set the storage engine at server startup, use the --default-storage-engine option. See Section 5.1.2, “Server Command Options”.

  • sync_frm

    Command Line Format --sync-frm
    Config File Format sync-frm
    Option Sets Variable Yes, sync_frm
    Variable Name sync_frm
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default TRUE

    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.

  • system_time_zone

    Variable Name system_time_zone
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type string

    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”.

  • table_cache

    Version Removed 5.1.3
    Version Deprecated 5.1.3
    Command Line Format --table_cache=#
    Config File Format table_cache
    Option Sets Variable Yes, table_cache
    Variable Name table_cache
    Variable Scope Global
    Dynamic Variable Yes
    Deprecated 5.1.3, by table_open_cache
    Value Set
    Type numeric
    Default 64
    Range 1-524288

    This is the old name of table_open_cache before MySQL 5.1.3. From 5.1.3 on, use table_open_cache instead.

  • table_definition_cache

    Version Introduced 5.1.3
    Command Line Format --table_definition_cache=#
    Config File Format table_definition_cache
    Option Sets Variable Yes, table_definition_cache
    Variable Name table_definition_cache
    Variable Scope Global
    Dynamic Variable Yes
    Value Set (<= 5.1.24)
    Type numeric
    Default 128
    Range 1-524288
    Value Set (>= 5.1.25)
    Type numeric
    Default 256
    Range 256-524288

    The number of table definitions that can be stored in the definition cache. If you use a large number of tables, you can create a large table definition cache to speed up opening of tables. The table definition cache takes less space and does not use file descriptors, unlike the normal table cache. This variable was added in MySQL 5.1.3. The minimum and default values are 1 and 128 before MySQL 5.1.25. The minimum and default are both 256 as of MySQL 5.1.25.

  • table_lock_wait_timeout

    Command Line Format --table_lock_wait_timeout=#
    Config File Format table_lock_wait_timeout
    Option Sets Variable Yes, table_lock_wait_timeout
    Variable Name table_lock_wait_timeout
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 50
    Range 1-1073741824

    This variable currently is unused.

  • table_open_cache

    Version Introduced 5.1.3
    Command Line Format --table-open-cache=#
    Config File Format table_open_cache
    Variable Name table_open_cache
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 64
    Range 1-524288

    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, “Server Status Variables”. If the value of Opened_tables is large and you don't do FLUSH TABLES often (which just forces all tables to be closed and reopened), then you should increase the value of the table_open_cache variable. For more information about the table cache, see Section 7.4.7, “How MySQL Opens and Closes Tables”. Before MySQL 5.1.3, this variable is called table_cache.

  • table_type

    Variable Name table_type
    Variable Scope Both
    Dynamic Variable Yes
    Deprecated 5.2.5, by storage_engine
    Value Set
    Type enumeration

    This variable is a synonym for storage_engine. In MySQL 5.1, storage_engine is the preferred name. In MySQL 6.0, table_type will be removed.

  • thread_cache_size

    Command Line Format --thread_cache_size=#
    Config File Format thread_cache_size
    Option Sets Variable Yes, thread_cache_size
    Variable Name thread_cache_size
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 0
    Range 0-16384

    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, “Server Status Variables”.

  • thread_concurrency

    Command Line Format --thread_concurrency=#
    Config File Format thread_concurrency
    Option Sets Variable Yes, thread_concurrency
    Variable Name thread_concurrency
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type numeric
    Default 10
    Range 1-512

    This variable is specific to Solaris systems, for which mysqld invokes the thr_setconcurrency() with the variable 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.

  • thread_handling

    Version Introduced 5.1.17
    Command Line Format --thread_handling=name
    Config File Format thread_handling
    Option Sets Variable Yes, thread_handling
    Variable Name thread_handling
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type enumeration
    Valid Values no-threads, one-thread-per-connection

    The thread-handling model. The allowable values are one-thread (the server uses one thread) and one-thread-per-connection (the server uses one thread to handle each client connection). one-thread is useful for debugging under Linux; see MySQL Internals: Porting. This variable was added in MySQL 5.1.17

  • thread_stack

    Command Line Format --thread_stack=#
    Config File Format thread_stack
    Option Sets Variable Yes, thread_stack
    Variable Name thread_stack
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 196608
    Range 131072-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 196608
    Range 131072-18446744073709547520

    The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. See Section 7.1.4, “The MySQL Benchmark Suite”. The default (192KB) is large enough for normal operation. 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

    Command Line Format --default_time_zone=string
    Config File Format default_time_zone
    Variable Name time_zone
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type string

    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”.

  • timed_mutexes

    Command Line Format --timed_mutexes
    Config File Format timed_mutexes
    Option Sets Variable Yes, timed_mutexes
    Variable Name timed_mutexes
    Variable Scope Global
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default OFF

    This variable controls whether InnoDB mutexes are timed. If this variable is set to 0 or OFF (the default), mutex timing is disabled. If the variable is set to 1 or ON, mutex timing is enabled. With timing enabled, the os_wait_times value in the output from SHOW ENGINE INNODB MUTEX indicates the amount of time (in ms) spent in operating system waits. Otherwise, the value is 0.

  • tmp_table_size

    Version Removed 5.1.12
    Command Line Format --tmp_table_size=#
    Config File Format tmp_table_size
    Option Sets Variable Yes, tmp_table_size
    Variable Name tmp_table_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default system dependent
    Range 1024-4294967295

    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

    Command Line Format --tmpdir=name
    Config File Format tmpdir
    Option Sets Variable Yes, tmpdir
    Variable Name tmpdir
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type filename

    The directory used for temporary files and temporary tables. 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 file system 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.

  • transaction_alloc_block_size

    Command Line Format --transaction_alloc_block_size=#
    Config File Format transaction_alloc_block_size
    Option Sets Variable Yes, transaction_alloc_block_size
    Variable Name transaction_alloc_block_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 8192
    Range 1024-4294967295

    The amount in bytes by which to increase a per-transaction memory pool which needs memory. See the description of transaction_prealloc_size.

  • transaction_prealloc_size

    Command Line Format --transaction_prealloc_size=#
    Config File Format transaction_prealloc_size
    Option Sets Variable Yes, transaction_prealloc_size
    Variable Name transaction_prealloc_size
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Platform Bit Size 32
    Type numeric
    Default 4096
    Range 1024-4294967295
    Value Set
    Platform Bit Size 64
    Type numeric
    Default 4096
    Range 1024-18446744073709547520

    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.

  • tx_isolation

    Variable Name tx_isolation
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type enumeration
    Default REPEATABLE-READ
    Valid Values READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, SERIALIZABLE

    The default transaction isolation level. Defaults to REPEATABLE-READ.

    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.

  • updatable_views_with_limit

    Command Line Format --updatable_views_with_limit=#
    Config File Format updatable_views_with_limit
    Option Sets Variable Yes, updatable_views_with_limit
    Variable Name updatable_views_with_limit
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type boolean
    Default 1

    This variable controls whether updates to a view can be made when the view does not contain all columns of the primary key defined in the underlying table, if the update statement contains a LIMIT clause. (Such updates often are generated by GUI tools.) An update is an UPDATE or DELETE statement. Primary key here means a PRIMARY KEY, or a UNIQUE index in which no column can contain NULL.

    The variable can have two values:

    • 1 or YES: Issue a warning only (not an error message). This is the default value.

    • 0 or NO: Prohibit the update.

  • version

    The version number for the server.

  • 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.

  • version_compile_machine

    The type of machine or architecture on which MySQL was built.

  • version_compile_os

    Variable Name version_compile_os
    Variable Scope Global
    Dynamic Variable No
    Value Set
    Type string

    The type of operating system on which MySQL was built.

  • wait_timeout

    Command Line Format --wait_timeout=#
    Config File Format wait_timeout
    Option Sets Variable Yes, wait_timeout
    Variable Name wait_timeout
    Variable Scope Both
    Dynamic Variable Yes
    Value Set
    Type numeric
    Default 28800
    Range 1-31536000
    Value Set
    Type (windows) numeric
    Default 28800
    Range 1-2147483

    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.

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