Performance Schema System Variable Tables •Index on ( OWNER_THREAD_ID ,  OWNER_EVENT_ID ) TRUNCATE TABLE  is not permitted for the  table_handles  table. 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 • VARIABLE_VALUE •Primary key on ( VARIABLE_NAME ) • THREAD_ID • VARIABLE_NAME • VARIABLE_VALUE The session variable value for the session named by the  THREAD_ID  column. The system variable name. The thread identifier of the session in which the system variable is defined. The  variables_by_thread  table has these columns: The  global_variables  and  session_variables  tables have these indexes: 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 system variable name. 5031