• MySQL 5.1 Reference Manual :: 5 MySQL Server Administration :: 5.1 The MySQL Server :: 5.1.4 Server System Variables
  • 5.1.4. 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://dev.mysql.com/doc/refman/5.0/en/, and http://dev.mysql.com/doc/refman/4.1/en/.

    The following table lists all available system variables:

    Table 5.2. 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_direct_non_transactional_updates Yes Yes Yes Both 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 No
    datetime_format Yes Yes Yes Both No
    debug Yes Yes Yes Both Yes
    debug_sync     Yes Both Yes
    default-storage-engine 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_community_features     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_flushing 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_change_buffering Yes Yes Yes Global Yes
    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_format Yes Yes Yes Global Yes
    innodb_file_format_check 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_io_capacity Yes Yes Yes Global No
    innodb_lock_wait_timeout Yes Yes Yes Both Yes
    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_old_blocks_pct Yes Yes Yes Global Yes
    innodb_old_blocks_time Yes Yes Yes Global Yes
    innodb_open_files Yes Yes Yes Global No
    innodb_read_ahead_threshold Yes Yes Yes Global Yes
    innodb_read_io_threads Yes Yes Yes Global No
    innodb_replication_delay Yes Yes Yes Global Yes
    innodb_rollback_on_timeout Yes Yes Yes Global No
    innodb_spin_wait_delay Yes Yes Yes Global Yes
    innodb_stats_on_metadata Yes Yes Yes Global Yes
    innodb_stats_sample_pages Yes Yes Yes Global Yes
    innodb_strict_mode Yes Yes Yes Both 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
    innodb_use_sys_malloc Yes Yes Yes Global No
    innodb_version     Yes Global No
    innodb_write_io_threads Yes Yes Yes Global No
    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_files_support     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 Global 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_mmap_size Yes Yes Yes Global No
    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_bin Yes   Yes Both Yes
    ndb_log_binlog_index Yes   Yes Global 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_use_transactions Yes Yes Yes Both Yes
    ndb_wait_connected Yes Yes Yes   No
    ndb-wait-setup Yes Yes Yes   No
    ndbinfo_database     Yes   No
    ndbinfo_max_bytes Yes   Yes Both Yes
    ndbinfo_max_rows Yes   Yes Both Yes
    ndbinfo_show_hidden Yes   Yes Both Yes
    ndbinfo_table_prefix Yes   Yes Both Yes
    ndbinfo_version     Yes Global 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-alter-table Yes Yes     Yes
    - Variable: old_alter_table     Yes Both Yes
    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
    partition Yes Yes     No
    - Variable: have_partitioning     Yes Global No
    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
    pseudo_thread_id     Yes Session Yes
    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-name-resolve Yes Yes 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
    slave_type_conversions Yes Yes Yes Global No
    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 Both 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 Global Yes
    sync_frm Yes Yes 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 No
    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.

    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.

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

    Some system variables take file name values. Unless otherwise specified, the default file location is the data directory if the value is a relative path name. To specify the location explicitly, use an absolute path name. Suppose that the data directory is /var/mysql/data. If a file-valued variable is given as a relative path name, it will be located under /var/mysql/data. If the value is an absolute path name, its location is as given by the path name.

    • autocommit

      The autocommit mode. If set to 1, all changes to a table take effect immediately. If set to 0, you must use COMMIT to accept a transaction or ROLLBACK to cancel it. By default, client connections begin with autocommit set to 1. If you change autocommit mode from 0 to 1, MySQL performs an automatic COMMIT of any open transaction. Another way to begin a transaction is to use a START TRANSACTION or BEGIN statement. See Section 12.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.

    • automatic_sp_privileges

      Variable Name automatic_sp_privileges
      Variable Scope Global
      Dynamic Variable Yes
        Permitted Values
      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 from the creator when the routine is dropped. If automatic_sp_privileges is 0, the server does not automatically add or drop these privileges.

      The creator of a routine is the account used to execute the CREATE statement for it. This might not be the same as the account named as the DEFINER in the routine definition.

      See also Section 19.2.2, “Stored Routines and MySQL 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
        Permitted Values
      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
        Permitted Values
      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.

    • big_tables

      If set to 1, all temporary tables are stored on disk rather than in memory. This is a little slower, but the error The table tbl_name is full does not occur for SELECT operations that require a large temporary table. The default value for a new connection is 0 (use in-memory temporary tables). Normally, you should never need to set this variable, because in-memory tables are automatically converted to disk-based tables as required.

      Note

      This variable was formerly named sql_big_tables.

    • 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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 8388608
      Range 0-4294967295
        Permitted Values
      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 nonempty 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
        Permitted Values
      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
        Permitted Values
      Type string

      The character set for the current connection. Used for literals that do not have an explicit character set specification 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.
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      Type string

      The character set used for returning query results such as result sets or error messages 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
        Permitted Values
      Type string

      The server's default character set.

    • character_set_system

      Variable Name character_set_system
      Variable Scope Global
      Dynamic Variable No
        Permitted Values
      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
        Permitted Values
      Type filename

      The directory where character sets are installed.

    • collation_connection

      Variable Name collation_connection
      Variable Scope Both
      Dynamic Variable Yes
        Permitted Values
      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.
        Permitted Values
      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
        Permitted Values
      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

      The transaction completion type. This variable can take the values shown in the following table.

      Value Description
      0 COMMIT and ROLLBACK are unaffected. This is the default value.
      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.)
      2 COMMIT and ROLLBACK are equivalent to COMMIT RELEASE and ROLLBACK RELEASE, respectively. (The server disconnects after terminating the transaction.)

      completion_type affects transactions that begin with START TRANSACTION or BEGIN and end with COMMIT or ROLLBACK. It does not apply to implicit commits resulting from execution of the statements listed in Section 12.3.3, “Statements That Cause an Implicit Commit”. It also does not apply for XA COMMIT, XA ROLLBACK, or when autocommit=1.

    • 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
        Permitted Values
      Type boolean
      Default TRUE

      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. If you start mysqld with --skip-new, this variable is set to 0.

      This variable can take three integer values.

      Value Description
      0 Disables concurrent inserts
      1 (Default) Enables concurrent insert for MyISAM tables that do not 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
        Permitted Values (<= 5.1.22)
      Type numeric
      Default 5
        Permitted Values (>= 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
        Permitted Values
      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
        Permitted Values
      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.

    • debug_sync

      Version Introduced 5.1.41
      Variable Name debug_sync
      Variable Scope Both
      Dynamic Variable Yes
        Permitted Values
      Type string

      This variable is the user interface to the Debug Sync facility. Use of Debug Sync requires that MySQL be configured with the --enable-debug-sync option (see Section 2.3.2, “Typical configure Options”). If Debug Sync is not compiled in, this system variable is not available.

      The global variable value is read only and indicates whether the facility is enabled. By default, Debug Sync is disabled and the value of debug_sync is OFF. If the server is started with --debug-sync-timeout=N, where N is a timeout value greater than 0, Debug Sync is enabled and the value of debug_sync is ON - current signal followed by the signal name. Also, N becomes the default timeout for individual synchronization points.

      The session value can be read by any user and will have the same value as the global variable. The session value can be set by users that have the SUPER privilege to control synchronization points.

      For a description of the Debug Sync facility and how to use synchronization points, see MySQL Internals: Test Synchronization.

      This variable was added in MySQL 5.1.41.

    • 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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 100
      Range 1-4294967295
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 1000
      Range 1-4294967295
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values (>= 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 nonindexed 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”.

    • error_count

      The number of errors that resulted from the last statement that generated messages. This variable is read only. See Section 12.4.5.18, “SHOW ERRORS Syntax”.

    • 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
        Permitted Values
      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
        Permitted Values
      Type numeric
      Default 0
      Range 0-99

      The number of days for automatic binary log file removal. The default is 0, which means “no automatic removal.” Possible removals happen at startup and when the binary log is flushed. Log flushing occurs as indicated in Section 5.2, “MySQL Server Logs”.

      To remove binary log files manually, use the PURGE BINARY LOGS statement. See Section 12.5.1.1, “PURGE BINARY LOGS Syntax”.

    • flush

      Command-Line Format --flush
      Config-File Format flush
      Variable Name flush
      Variable Scope Global
      Dynamic Variable Yes
        Permitted Values
      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.5.4.2, “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
        Permitted Values
      Type numeric
      Default 0
      Min Value 0
        Permitted Values
      Type (windows) numeric
      Default 1800
      Min Value 0

      If this is set to a nonzero value, all tables are closed every flush_time seconds to free up resources and synchronize unflushed data to disk. This option is best used only on Windows 9x or Me, or on systems with minimal resources.

    • foreign_key_checks

      If set to 1 (the default), foreign key constraints for InnoDB tables are checked. If set to 0, they are ignored. Disabling foreign key checking can be useful for reloading InnoDB tables in an order different from that required by their parent/child relationships. See Section 13.6.4.4, “FOREIGN KEY Constraints”.

      Setting foreign_key_checks to 0 also affects data definition statements: DROP SCHEMA drops a schema even if it contains tables that have foreign keys that are referred to by tables outside the schema, and DROP TABLE drops tables that have foreign keys that are referred to by other tables.

      Note

      Setting foreign_key_checks to 1 does not trigger a scan of the existing table data. Therefore, rows added to the table while foreign_key_checks = 0 will not be verified for consistency.

    • 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
        Permitted Values
      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 nonalphanumeric 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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      Type filename

      The file from which to read the list of stopwords for full-text searches. The server looks for the file in the data directory unless an absolute path name is given to specify a different directory. 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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 1024
      Range 4-4294967295
        Permitted Values
      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_community_features

      YES if statement profiling is enabled, NO if not. See Section 12.4.5.32, “SHOW PROFILE Syntax”.

      This variable is renamed to have_profiling in MySQL 5.5.

    • 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
        Permitted Values
      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
        Permitted Values
      Type string

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

    • identity

      This variable is a synonym for the last_insert_id variable. It exists for compatibility with other database systems. You can read its value with SELECT @@identity, and set it using SET identity.

    • 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
        Permitted Values
      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
        Permitted Values
      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. No statement terminator such as ;, \g, or \G should be given at the end of each statement.

      Note that the --init-file option is unavailable if MySQL was configured with the --disable-grant-options option. See Section 2.3.2, “Typical configure Options”.

    • innodb_xxx

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

    • insert_id

      The value to be used by the following INSERT or ALTER TABLE statement when inserting an AUTO_INCREMENT value. This is mainly used with the binary log.

    • 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
        Permitted Values
      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

      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
        Permitted Values
      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
        Permitted Values (<= 5.1.22)
      Type numeric
      Default 8388608
      Range 8-4294967295
        Permitted Values (>= 5.1.23)
      Platform Bit Size 32
      Type numeric
      Default 8388608
      Range 8-4294967295
        Permitted Values (>= 5.1.23)
      Platform Bit Size 64
      Type numeric
      Default 8388608
      Range 8-OS_PER_PROCESS_LIMIT

      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.

      You can increase the value to get better index handling for all reads and multiple writes; on a system whose primary function is to run MySQL using the MyISAM storage engine, 25% of the machine's total memory is an acceptable value for this variable. However, you should be aware that, if you make the value too large (for example, more than 50% of the machine's total memory), your system might start to page and become extremely slow. This is because 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. You should also consider the memory requirements of any other storage engines that you may be using in addition to MyISAM.

      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 12.4.5, “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 is allocated internally for administrative structures. Factors that influence the amount of overhead for these structures include block size and pointer size. As block size increases, the percentage of the key buffer lost to overhead tends to decrease. Larger blocks results in a smaller number of read operations (because more keys are obtained per read), but conversely an increase in reads of keys that are not examined (if not all keys in a block are relevant to a query).

      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 300
      Range 100-4294967295
        Permitted Values
      Platform Bit Size 64
      Type numeric
      Default 300
      Range 100-18446744073709547520

      This value controls the demotion of buffers from the hot sublist of a key cache to the warm sublist. 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
        Permitted Values
      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
        Permitted Values
      Type numeric
      Default 100
      Range 1-100

      The division point between the hot and warm sublists of the key cache buffer list. The value is the percentage of the buffer list to use for the warm sublist. 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
      Deprecated 5.5.0, by lc-messages-dir
        Permitted Values
      Type filename
      Default /usr/local/mysql/share/mysql/english/

      The directory where error messages are located. See Section 9.2, “Setting the Error Message Language”.

    • 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
        Permitted Values
      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
        Permitted Values
      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”.

    • last_insert_id

      The value to be returned from LAST_INSERT_ID(). This is stored in the binary log when you use LAST_INSERT_ID() in a statement that updates a table. Setting this variable does not update the value returned by the mysql_insert_id() C API function.

    • lc_time_names

      Version Introduced 5.1.12
      Variable Name lc_time_names
      Variable Scope Both
      Dynamic Variable Yes
        Permitted Values
      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 9.7, “MySQL Server Locale Support”. This variable was added in MySQL 5.1.12.

    • license

      Variable Name license
      Variable Scope Global
      Dynamic Variable No
        Permitted Values
      Type string
      Default GPL

      The type of license the server has.

    • local_infile

      Variable Name local_infile
      Variable Scope Global
      Dynamic Variable Yes
        Permitted Values
      Type boolean

      Whether LOCAL is supported for LOAD DATA INFILE statements. See Section 5.3.5, “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
        Permitted Values
      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 19.7, “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
        Permitted Values
      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
        Permitted Values
      Type set
      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

      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
        Permitted Values
      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

      Version Deprecated 5.1.29
      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
      Deprecated 5.1.29, by slow-query-log
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 64
      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. As of MySQL 5.1.38, the server logs messages about statements that are unsafe for statement-based logging only if the value is greater than 0.

    • 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
        Permitted Values (>= 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
        Permitted Values
      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 (such as MyISAM, MEMORY, and 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
        Permitted Values
      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
        Permitted Values
      Type numeric
      Default 0
      Range 0-2

      If set to 0, table names are stored as specified and comparisons are case sensitive. If set to 1, table names are stored in lowercase on disk and 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. For additional information, see Section 8.2.2, “Identifier Case Sensitivity”.

      You should not set this variable to 0 if you are running MySQL on a system that has case-insensitive file names (such as Windows or Mac OS X). If you set this variable to 0 on such a system and access MyISAM tablenames using different lettercases, index corruption may result. On Windows the default value is 1. On Mac OS X, the default value is 2.

      If you are using InnoDB tables, you should set this variable to 1 on all platforms to force names to be converted to lowercase. Prior to MySQL Cluster NDB 6.3.35, MySQL Cluster NDB 7.0.15, and MySQL Cluster NDB 7.1.4, this is also true for tables using the NDB storage engine.

    • 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 Global
      Dynamic Variable Yes
        Permitted Values
      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; nonmultiples 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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 10
      Range 1-4294967295
        Permitted Values
      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. If a connection is established successfully within fewer than max_connect_errors attempts after a previous connection was interrupted, the error count for the host is cleared to zero. However, once a host is blocked, the FLUSH HOSTS statement is the only way to unblock it.

    • 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
        Permitted Values (<= 5.1.14)
      Type numeric
      Default 100
        Permitted Values (>= 5.1.15)
      Type numeric
      Default 151
      Range 1-16384
        Permitted Values (>= 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.5.2.7, “Too many connections, for more information.

      MySQL Enterprise.  For notification that the maximum number of connections is getting dangerously high and for advice on setting the optimum value for max_connections subscribe to the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

      Increasing this value increases the number of file descriptors that mysqld requires. See Section 7.4.8, “How MySQL Opens and Closes Tables”, for comments on file descriptor limits.

    • max_delayed_threads

      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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 16.2.1, “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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 4294967295
      Range 1-4294967295
        Permitted Values
      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 12.4.5.23, “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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 32
      Range 1-4294967295
        Permitted Values
      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 variable 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
        Permitted Values
      Type numeric
      Default 0
      Range 0-4294967295

      The maximum number of simultaneous connections allowed to any given MySQL user account. A value of 0 (the default) means “no limit.

      This variable has a global value that can be set at server startup or runtime. It also has a read-only session value that indicates the effective simultaneous-connection limit that applies to the account associated with the current session. The session value is initialized as follows:

      • If the user account has a nonzero MAX_USER_CONNECTIONS resource limit, the session max_user_connections value is set to that limit.

      • Otherwise, the session max_user_connections value is set to the global value.

      Account resource limits are specified using the GRANT statement. See Section 5.5.4, “Setting Account Resource Limits”, and Section 12.4.1.3, “GRANT Syntax”.

    • 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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 4294967295
      Range 1-4294967295
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 0
      Range 0-4294967295
        Permitted Values
      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
        Permitted Values
      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.5.2.12, “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
        Permitted Values
      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_mmap_size

      Version Introduced 5.1.43
      Command-Line Format --myisam_mmap_size=#
      Config-File Format myisam_mmap_size
      Option Sets Variable Yes, myisam_mmap_size
      Variable Name myisam_mmap_size
      Variable Scope Global
      Dynamic Variable No
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 4294967295
      Range 7-4294967295
        Permitted Values
      Platform Bit Size 64
      Type numeric
      Default 18446744073709547520
      Range 7-18446744073709547520

      The maximum amount of memory to use for memory mapping compressed MyISAM files. If many compressed MyISAM tables are used, the value can be decreased to reduce the likelihood of memory-swapping problems. This variable was added in MySQL 5.1.43.

    • 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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 1
      Range 1-4294967295
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 8388608
      Range 4-4294967295
        Permitted Values
      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
        Permitted Values
      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 7.4.7, “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
        Permitted Values
      Type boolean
      Default OFF

      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 10
      Range 1-4294967295
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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_alter_table

      Command-Line Format --old-alter-table
      Config-File Format old-alter-table
      Option Sets Variable Yes, old_alter_table
      Variable Name old-alter-table
      Variable Scope Both
      Dynamic Variable Yes
        Permitted Values
      Type boolean
      Default OFF

      When this variable is enabled, the server does not use the optimized method of processing an ALTER TABLE operation. It reverts to using a temporary table, copying over the data, and then renaming the temporary table to the original, as used by MySQL 5.0 and earlier. For more information on the operation of ALTER TABLE, see Section 12.1.7, “ALTER TABLE Syntax”.

    • 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
        Permitted Values
      Type boolean
      Default FALSE

      Whether the server should use pre-4.1-style passwords for MySQL user accounts. See Section B.5.2.4, “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 12.4.4, “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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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 63, the optimizer switches to the algorithm used in MySQL 5.0.0 (and previous versions) for performing searches. The value of 63 is deprecated and will be treated as invalid in a future MySQL release.

    • 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

      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
        Permitted Values
      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
        Permitted Values
      Type filename
      Default /usr/local/mysql/lib/mysql

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

      If the plugin directory is writable by the server, it may be possible for a user to write executable code to a file in the directory using SELECT ... INTO DUMPFILE. This can be prevented by making plugin_dir read only to the server or by setting --secure-file-priv to a directory where SELECT writes can be made safely.

    • port

      Command-Line Format --port=#
      Config-File Format port
      Option Sets Variable Yes, port
      Variable Name port
      Variable Scope Global
      Dynamic Variable No
        Permitted Values
      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
        Permitted Values
      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.

    • profiling

      If set to 0 (the default), statement profiling is disabled. If set to 1, statement profiling is enabled and the SHOW PROFILES and SHOW PROFILE statements provide access to profiling information. See Section 12.4.5.33, “SHOW PROFILES Syntax”. This variable was added in MySQL 5.1.24.

    • profiling_history_size

      The number of statements for which to maintain profiling information if profiling is enabled. The default value is 15. The maximum value is 100. Setting the value to 0 effectively disables profiling. See Section 12.4.5.33, “SHOW PROFILES Syntax”. This variable was added in MySQL 5.1.24.

    • protocol_version

      Variable Name protocol_version
      Variable Scope Global
      Dynamic Variable No
        Permitted Values
      Type numeric

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

    • pseudo_thread_id

      Variable Name pseudo_thread_id
      Variable Scope Session
      Dynamic Variable Yes
        Permitted Values
      Type numeric

      This variable is for internal server use.

    • 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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 8192
      Range 1024-4294967295
      Block Size 1024
        Permitted Values
      Platform Bit Size 64
      Type numeric
      Default 8192
      Range 1024-18446744073709547520
      Block Size 1024

      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 parameter.

    • 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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 1048576
      Range 0-4294967295
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 4096
      Range 512-4294967295
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 0
      Range 0-4294967295
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 8192
      Range 8192-4294967295
      Block Size 1024
        Permitted Values
      Platform Bit Size 64
      Type numeric
      Default 8192
      Range 8192-18446744073709547520
      Block Size 1024

      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.

    • rand_seed1

      The rand_seed1 and rand_seed2 variables exist as session variables only, and can be set but not read. Beginning with MySQL 5.1.18, the variables—but not their values—are shown in the output of SHOW VARIABLES.

      The purpose of these variables is to support replication of the RAND() function. For statements that invoke RAND(), the master passes two values to the slave, where they are used to seed the random number generator. The slave uses these values to set the session variables rand_seed1 and rand_seed2 so that RAND() on the slave generates the same value as on the master.

    • rand_seed2

      See the description for rand_seed1.

    • 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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 4096
      Range 4096-4294967295
      Block Size 1024

      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
        Permitted Values
      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
        Permitted Values
      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. In replication setups, it can be useful to enable read_only on slave servers to ensure that slaves accept updates only from the master server and not from clients.

      read_only 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”). This variable does not prevent the use of ANALYZE TABLE or OPTIMIZE TABLE statements because its purpose is to prevent changes to table structure or contents. Analysis and optimization do not qualify as such changes. This means, for example, that consistency checks on read-only slaves can be performed with mysqlcheck --all-databases --analyze.

      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 0
      Range 0-4294967295
        Permitted Values
      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
        Permitted Values
      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.5.2.4, “Client does not support authentication protocol.

    • secure_file_priv

      Version Introduced 5.1.17
      Command-Line Format --secure-file-priv=path
      Config-File Format secure-file-priv=path
      Option Sets Variable Yes, secure_file_priv
      Variable Name secure-file-priv
      Variable Scope Global
      Dynamic Variable No
        Permitted Values
      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
        Permitted Values
      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_name_resolve

      This variable is set from the value of the --skip-name-resolve option. If it is ON, mysqld resolves host names when checking client connections. If OFF, mysqld uses only IP numbers and all Host column values in the grant tables must be IP numbers or localhost. See Section 7.5.11, “How MySQL Uses DNS”.

      This variable was added in MySQL 5.1.46.

    • 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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 2097144
      Max Value 4294967295
        Permitted Values
      Platform Bit Size 64
      Type numeric
      Default 2097144
      Max Value 18446744073709547520

      Each session that needs to do a sort allocates a buffer of this size. If you see many Sort_merge_passes per second in SHOW GLOBAL STATUS output, you can consider increasing this to speed up ORDER BY or GROUP BY operations that cannot be improved with query optimization or improved indexing. The entire buffer is allocated even if it is not all needed, so setting it larger than required globally will slow down most queries that sort. It is best to increase it as a session setting, and only for the sessions that need a larger size. On Linux, there are thresholds of 256KB and 2MB where larger values may significantly slow down memory allocation, so you should consider staying below one of those values. Experiment to find the best value for your workload. See Section B.5.4.4, “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_auto_is_null

      If this variable is set to 1 (the default), then after a statement that successfully inserts an automatically generated AUTO_INCREMENT value, you can find that value by issuing a statement of the following form:

      SELECT * FROM tbl_name WHERE auto_col IS NULL
      

      If the statement returns a row, the value returned is the same as if you invoked the LAST_INSERT_ID() function. For details, including the return value after a multiple-row insert, see Section 11.11.3, “Information Functions”. If no AUTO_INCREMENT value was successfully inserted, the SELECT statement returns no row.

      The behavior of retrieving an AUTO_INCREMENT value by using an IS NULL comparison is used by some ODBC programs, such as Access. See Section 21.1.7.1.1, “Obtaining Auto-Increment Values”. This behavior can be disabled by setting sql_auto_is_null to 0.

    • sql_big_selects

      If set to 0, MySQL aborts SELECT statements that are likely to take a very long time to execute (that is, statements for which the optimizer estimates that the number of examined rows exceeds the value of max_join_size). This is useful when an inadvisable WHERE statement has been issued. The default value for a new connection is 1, which allows all SELECT statements.

      If you set the max_join_size system variable to a value other than DEFAULT, sql_big_selects is set to 0.

    • sql_buffer_result

      If set to 1, sql_buffer_result forces results from SELECT statements to be put into temporary tables. This helps MySQL free the table locks early and can be beneficial in cases where it takes a long time to send results to the client. The default value is 0.

    • sql_log_bin

      If set to 0, no logging is done to the binary log for the client. The client must have the SUPER privilege to set this option. The default value is 1.

    • sql_log_off

      If set to 1, no logging is done to the general query log for this client. The client must have the SUPER privilege to set this option. The default value is 0.

    • sql_log_update

      This variable is deprecated, and is mapped to sql_log_bin. It is removed in MySQL 5.5.

    • 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
        Permitted Values
      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_notes

      If set to 1 (the default), warnings of Note level are recorded. If set to 0, Note warnings are suppressed. mysqldump includes output to set this variable to 0 so that reloading the dump file does not produce warnings for events that do not affect the integrity of the reload operation.

    • sql_quote_show_create

      If set to 1 (the default), the server quotes identifiers for SHOW CREATE TABLE and SHOW CREATE DATABASE statements. If set to 0, quoting is disabled. This option is enabled by default so that replication works for identifiers that require quoting. See Section 12.4.5.12, “SHOW CREATE TABLE Syntax”, and Section 12.4.5.8, “SHOW CREATE DATABASE Syntax”.

    • sql_safe_updates

      If set to 1, MySQL aborts UPDATE or DELETE statements that do not use a key in the WHERE clause or a LIMIT clause. This makes it possible to catch UPDATE or DELETE statements where keys are not used properly and that would probably change or delete a large number of rows. The default value is 0.

    • sql_select_limit

      Variable Name sql_select_limit
      Variable Scope Both
      Dynamic Variable Yes
        Permitted Values
      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.

    • sql_warnings

      This variable controls whether single-row INSERT statements produce an information string if warnings occur. The default is 0. Set the value to 1 to produce an information string.

    • 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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      Type boolean
      Default TRUE

      If this variable is set to 1, when any nontemporary 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
        Permitted Values
      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 9.6, “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
        Permitted Values
      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
        Permitted Values (<= 5.1.24)
      Type numeric
      Default 128
      Range 1-524288
        Permitted Values (>= 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
        Permitted Values
      Type numeric
      Default 50
      Range 1-1073741824

      This variable 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
        Permitted Values
      Type numeric
      Default 64
      Range 64-524288

      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 7.4.8, “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
        Permitted Values
      Type enumeration

      This variable is a synonym for storage_engine. In MySQL 5.1, storage_engine is the preferred name; table_type is deprecated and is removed in MySQL 5.5.

    • 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
        Permitted Values
      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 does not provide a notable performance improvement if you have a good thread implementation. However, if your server sees hundreds of connections per second you should normally set thread_cache_size high enough so that most new connections use cached threads. 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
        Permitted Values
      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

      The thread-handling model. The allowable values are no-threads (the server uses one thread) and one-thread-per-connection (the server uses one thread to handle each client connection). no-threads 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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 196608
      Range 131072-4294967295
      Block Size 1024
        Permitted Values
      Platform Bit Size 64
      Type numeric
      Default 262144
      Range 131072-18446744073709547520
      Block Size 1024

      The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. See Section 7.1.3, “The MySQL Benchmark Suite”. The default of 192KB (256KB for 64-bit systems) 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
        Permitted Values
      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 9.6, “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
        Permitted Values
      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.

    • timestamp = {timestamp_value | DEFAULT}

      Set the time for this client. This is used to get the original timestamp if you use the binary log to restore rows. timestamp_value should be a Unix epoch timestamp, not a MySQL timestamp.

      SET timestamp affects the value returned by NOW() but not by SYSDATE(). This means that timestamp settings in the binary log have no effect on invocations of SYSDATE(). The server can be started with the --sysdate-is-now option to cause SYSDATE() to be an alias for NOW(), in which case SET timestamp affects both functions.

    • 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
        Permitted Values
      Type numeric
      Default system dependent
      Range 1024-4294967295

      The maximum size of internal in-memory temporary tables. (The actual limit is determined as the minimum of tmp_table_size and max_heap_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.

      You can compare the number of internal on-disk temporary tables created to the total number of internal temporary tables created by comparing the values of the Created_tmp_disk_tables and Created_tmp_tables variables.

      See also Section 7.5.10, “How MySQL Uses Internal Temporary 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
        Permitted Values
      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. 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 nonpermanent 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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 8192
      Range 1024-4294967295
      Block Size 1024
        Permitted Values
      Platform Bit Size 64
      Type numeric
      Default 8192
      Range 1024-18446744073709547520
      Block Size 1024

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

    • transaction_allow_batching

      When set to 1 or ON, this variable enables batching of statements within the same transaction. In order to use this variable, autocommit must first be disabled by setting it to 0 or OFF; otherwise, setting transaction_allow_batching has no effect.

      This variable was added in MySQL Cluster NDB 6.3.7, and is currently supported for MySQL Cluster only.

    • 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
        Permitted Values
      Platform Bit Size 32
      Type numeric
      Default 4096
      Range 1024-4294967295
      Block Size 1024
        Permitted Values
      Platform Bit Size 64
      Type numeric
      Default 4096
      Range 1024-18446744073709547520
      Block Size 1024

      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
        Permitted Values
      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 12.3.6, “SET TRANSACTION Syntax”. If you set tx_isolation directly to an isolation level name that contains a space, the name should be enclosed within quotation marks, 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.

    • unique_checks

      If set to 1 (the default), uniqueness checks for secondary indexes in InnoDB tables are performed. If set to 0, storage engines are allowed to assume that duplicate keys are not present in input data. If you know for certain that your data does not contain uniqueness violations, you can set this to 0 to speed up large table imports to InnoDB.

      Note that setting this variable to 0 does not require storage engines to ignore duplicate keys. An engine is still allowed to check for them and issue duplicate-key errors if it detects them.

    • 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
        Permitted Values
      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
        Permitted Values
      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
        Permitted Values
      Type numeric
      Default 28800
      Range 1-31536000
        Permitted Values
      Type (windows) numeric
      Default 28800
      Range 1-2147483

      The number of seconds the server waits for activity on a noninteractive 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.

    • warning_count

      The number of errors, warnings, and notes that resulted from the last statement that generated messages. This variable is read only. See Section 12.4.5.42, “SHOW WARNINGS Syntax”.

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