27.12.14 Performance Schema System Variable Tables

The MySQL server maintains many system variables that indicate how it is configured (see Section 5.1.8, “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. See Section 27.12.14.1, “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. See Section 27.12.14.2, “Performance Schema variables_info Table”.

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

The global_variables and session_variables tables have these columns:

VARIABLE_NAME
The system variable name.
VARIABLE_VALUE
The system variable value. For global_variables, this column contains the global value. For session_variables, this column contains the variable value in effect for the current session.

The global_variables and session_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 the THREAD_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, the Performance_schema_thread_instances_lost status variable is greater than zero.

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