26.15 Performance Schema System Variables

performance_schema

The value of this variable is ON or OFF to indicate whether the Performance Schema is enabled. By default, the value is ON. At server startup, you can specify this variable with no value or a value of ON or 1 to enable it, or with a value of OFF or 0 to disable it.

Even when the Performance Schema is disabled, it continues to populate the global_variables, session_variables, global_status, and session_status tables. This occurs as necessary to permit the results for the SHOW VARIABLES and SHOW STATUS statements to be drawn from those tables, depending on the setting of the show_compatibility_56 system variable. The Performance Schema also populates some of the replication tables when disabled.

performance_schema_accounts_size

The number of rows in the accounts table. If this variable is 0, the Performance Schema does not maintain connection statistics in the accounts table or status variable information in the status_by_account table.

performance_schema_digests_size

The maximum number of rows in the events_statements_summary_by_digest table. If this maximum is exceeded such that a digest cannot be instrumented, the Performance Schema increments the Performance_schema_digest_lost status variable.

For more information about statement digesting, see Section 26.10, “Performance Schema Statement Digests and Sampling”.

performance_schema_error_size

The number of instrumented server error codes. The default value is the actual number of server error codes. Although the value can be set anywhere from 0 to its maximum, the intended use is to set it to either its default (to instrument all errors) or 0 (to instrument no errors).

Error information is aggregated in summary tables; see Section 26.12.17.11, “Error Summary Tables”. If an error occurs that is not instrumented, information for the occurrence is aggregated to the NULL row in each summary table; that is, to the row with ERROR_NUMBER=0, ERROR_NAME=NULL, and SQLSTATE=NULL.

performance_schema_events_stages_history_long_size

The number of rows in the events_stages_history_long table.

performance_schema_events_stages_history_size

The number of rows per thread in the events_stages_history table.

performance_schema_events_statements_history_long_size

The number of rows in the events_statements_history_long table.

performance_schema_events_statements_history_size

The number of rows per thread in the events_statements_history table.

performance_schema_events_transactions_history_long_size

The number of rows in the events_transactions_history_long table.

performance_schema_events_transactions_history_size

The number of rows per thread in the events_transactions_history table.

performance_schema_events_waits_history_long_size

The number of rows in the events_waits_history_long table.

performance_schema_events_waits_history_size

The number of rows per thread in the events_waits_history table.

performance_schema_hosts_size

The number of rows in the hosts table. If this variable is 0, the Performance Schema does not maintain connection statistics in the hosts table or status variable information in the status_by_host table.

performance_schema_max_cond_classes

The maximum number of condition instruments. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_cond_instances

The maximum number of instrumented condition objects. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_digest_length

The maximum number of bytes of memory reserved per statement for computation of normalized statement digest values in the Performance Schema. This variable is related to max_digest_length; see the description of that variable in Section 5.1.8, “Server System Variables”.

For more information about statement digesting, including considerations regarding memory use, see Section 26.10, “Performance Schema Statement Digests and Sampling”.

performance_schema_max_digest_sample_age

This variable affects statement sampling for the events_statements_summary_by_digest table. When a new table row is inserted, the statement that produced the row digest value is stored as the current sample statement associated with the digest. Thereafter, when the server sees other statements with the same digest value, it determines whether to use the new statement to replace the current sample statement (that is, whether to resample). Resampling policy is based on the comparative wait times of the current sample statement and new statement and, optionally, the age of the current sample statement:

For information about statement sampling, see Section 26.10, “Performance Schema Statement Digests and Sampling”.

performance_schema_max_file_classes

The maximum number of file instruments. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_file_handles

The maximum number of opened file objects. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

The value of performance_schema_max_file_handles should be greater than the value of open_files_limit: open_files_limit affects the maximum number of open file handles the server can support and performance_schema_max_file_handles affects how many of these file handles can be instrumented.

performance_schema_max_file_instances

The maximum number of instrumented file objects. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_index_stat

The maximum number of indexes for which the Performance Schema maintains statistics. If this maximum is exceeded such that index statistics are lost, the Performance Schema increments the Performance_schema_index_stat_lost status variable. The default value is autosized using the value of performance_schema_max_table_instances.

performance_schema_max_memory_classes

The maximum number of memory instruments. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_metadata_locks

The maximum number of metadata lock instruments. This value controls the size of the metadata_locks table. If this maximum is exceeded such that a metadata lock cannot be instrumented, the Performance Schema increments the Performance_schema_metadata_lock_lost status variable.

performance_schema_max_mutex_classes

The maximum number of mutex instruments. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_mutex_instances

The maximum number of instrumented mutex objects. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_prepared_statements_instances

The maximum number of rows in the prepared_statements_instances table. If this maximum is exceeded such that a prepared statement cannot be instrumented, the Performance Schema increments the Performance_schema_prepared_statements_lost status variable. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

The default value of this variable is autosized based on the value of the max_prepared_stmt_count system variable.

performance_schema_max_rwlock_classes

The maximum number of rwlock instruments. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_program_instances

The maximum number of stored programs for which the Performance Schema maintains statistics. If this maximum is exceeded, the Performance Schema increments the Performance_schema_program_lost status variable. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_rwlock_instances

The maximum number of instrumented rwlock objects. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_socket_classes

The maximum number of socket instruments. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_socket_instances

The maximum number of instrumented socket objects. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_sql_text_length

The maximum number of bytes used to store SQL statements. The value applies to storage required for these columns:

Any bytes in excess of performance_schema_max_sql_text_length are discarded and do not appear in the column. Statements differing only after that many initial bytes are indistinguishable in the column.

Decreasing the performance_schema_max_sql_text_length value reduces memory use but causes more statements to become indistinguishable if they differ only at the end. Increasing the value increases memory use but permits longer statements to be distinguished.

performance_schema_max_stage_classes

The maximum number of stage instruments. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_statement_classes

The maximum number of statement instruments. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

The default value is calculated at server build time based on the number of commands in the client/server protocol and the number of SQL statement types supported by the server.

This variable should not be changed, unless to set it to 0 to disable all statement instrumentation and save all memory associated with it. Setting the variable to nonzero values other than the default has no benefit; in particular, values larger than the default cause more memory to be allocated then is needed.

performance_schema_max_statement_stack

The maximum depth of nested stored program calls for which the Performance Schema maintains statistics. When this maximum is exceeded, the Performance Schema increments the Performance_schema_nested_statement_lost status variable for each stored program statement executed.

performance_schema_max_table_handles

The maximum number of opened table objects. This value controls the size of the table_handles table. If this maximum is exceeded such that a table handle cannot be instrumented, the Performance Schema increments the Performance_schema_table_handles_lost status variable. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_table_instances

The maximum number of instrumented table objects. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_table_lock_stat

The maximum number of tables for which the Performance Schema maintains lock statistics. If this maximum is exceeded such that table lock statistics are lost, the Performance Schema increments the Performance_schema_table_lock_stat_lost status variable.

performance_schema_max_thread_classes

The maximum number of thread instruments. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

performance_schema_max_thread_instances

The maximum number of instrumented thread objects. The value controls the size of the threads table. If this maximum is exceeded such that a thread cannot be instrumented, the Performance Schema increments the Performance_schema_thread_instances_lost status variable. For information about how to set and use this variable, see Section 26.7, “Performance Schema Status Monitoring”.

The max_connections system variable affects how many threads can run in the server. performance_schema_max_thread_instances affects how many of these running threads can be instrumented.

The variables_by_thread and status_by_thread tables contain system and status variable information only about foreground threads. If not all threads are instrumented by the Performance Schema, this table will miss some rows. In this case, the Performance_schema_thread_instances_lost status variable will be greater than zero.

performance_schema_session_connect_attrs_size

The amount of preallocated memory per thread reserved to hold connection attribute key-value pairs. If the aggregate size of connection attribute data sent by a client is larger than this amount, the Performance Schema truncates the attribute data, increments the Performance_schema_session_connect_attrs_lost status variable, and writes a message to the error log indicating that truncation occurred if the log_error_verbosity system variable is greater than 1. A _truncated attribute is also added to the session attributes with a value indicating how many bytes were lost, if the attribute buffer has sufficient space. This enables the Performance Schema to expose per-connection truncation information in the connection attribute tables. This information can be examined without having to check the error log.

The default value of performance_schema_session_connect_attrs_size is autosized at server startup. This value may be small, so if truncation occurs (Performance_schema_session_connect_attrs_lost becomes nonzero), you may wish to set performance_schema_session_connect_attrs_size explicitly to a larger value.

Although the maximum permitted performance_schema_session_connect_attrs_size value is 1MB, the effective maximum is 64KB because the server imposes a limit of 64KB on the aggregate size of connection attribute data it will accept. If a client attempts to send more than 64KB of attribute data, the server rejects the connection. For more information, see Section 26.12.9, “Performance Schema Connection Attribute Tables”.

performance_schema_setup_actors_size

The number of rows in the setup_actors table.

performance_schema_setup_objects_size

The number of rows in the setup_objects table.

performance_schema_users_size

The number of rows in the users table. If this variable is 0, the Performance Schema does not maintain connection statistics in the users table or status variable information in the status_by_user table.