29.12.14 Performance Schema System Variable Tables (original) (raw)

29.12.14 Performance Schema System Variable Tables

The MySQL server maintains many system variables that indicate how it is configured (seeSection 7.1.8, “Server System Variables”). System variable information is available in these Performance Schema tables:

• global_variables: Global system variables. An application that wants only global values should use this table.
• session_variables: System variables for the current session. An application that wants all system variable values for its own session should use this table. It includes the session variables for its session, as well as the values of global variables that have no session counterpart.
• variables_by_thread: Session system variables for each active session. An application that wants to know the session variable values for specific sessions should use this table. It includes session variables only, identified by thread ID.
• persisted_variables: Provides a SQL interface to the mysqld-auto.cnf file that stores persisted global system variable settings. SeeSection 29.12.14.1, “Performance Schema persisted_variables Table”.
• variables_info: Shows, for each system variable, the source from which it was most recently set, and its range of values. SeeSection 29.12.14.2, “Performance Schema variables_info Table”.

The SENSITIVE_VARIABLES_OBSERVER privilege is required to view the values of sensitive system variables in these tables.

The session variable tables (session_variables,variables_by_thread) contain information only for active sessions, not terminated sessions.

The global_variables andsession_variables tables have these columns:

• VARIABLE_NAME
  The system variable name.
• VARIABLE_VALUE
  The system variable value. Forglobal_variables, this column contains the global value. Forsession_variables, this column contains the variable value in effect for the current session.

The global_variables andsession_variables tables have these indexes:

• Primary key on (VARIABLE_NAME)

The variables_by_thread table has these columns:

• THREAD_ID
  The thread identifier of the session in which the system variable is defined.
• VARIABLE_NAME
  The system variable name.
• VARIABLE_VALUE
  The session variable value for the session named by theTHREAD_ID column.

The variables_by_thread table has these indexes:

• Primary key on (THREAD_ID,VARIABLE_NAME)

The variables_by_thread table contains system variable information only about foreground threads. If not all threads are instrumented by the Performance Schema, this table misses some rows. In this case, thePerformance_schema_thread_instances_lost status variable is greater than zero.

TRUNCATE TABLE is not supported for Performance Schema system variable tables.