Performance Schema System Variable Tables The  variables_by_thread  table has these indexes: 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. The  persisted_variables  table provides an SQL interface to the  mysqld-auto.cnf  file that stores persisted global system variable settings, enabling the file contents to be inspected at runtime using SELECT  statements. Variables are persisted using  SET PERSIST  or  PERSIST_ONLY  statements; see Section 13.7.6.1, “SET Syntax for Variable Assignment” . The table contains a row for each persisted system variable in the file. Variables not persisted do not appear in the table. For information about persisted system variables, see Section 13.7.6.1, “SET Syntax for Variable Assignment” . Suppose that  mysqld-auto.cnf  looks like this (slightly reformatted): Then  persisted_variables  has these contents: The  persisted_variables  table has these columns: The variable name listed in  mysqld-auto.cnf . • VARIABLE_NAME • VARIABLE_VALUE •Primary key on ( THREAD_ID ,  VARIABLE_NAME ) 27.12.14.1 Performance Schema persisted_variables Table { "Version": 1, "mysql_server": { "max_connections": { "Value": "1000", "Metadata": { "Timestamp": 1.519921706e+15, "User": "root", "Host": "localhost" } }, "autocommit": { "Value": "ON", "Metadata": { "Timestamp": 1.519921707e+15, "User": "root", "Host": "localhost" } } } } mysql>  SELECT * FROM performance_schema.persisted_variables; +-----------------+----------------+ | VARIABLE_NAME | VARIABLE_VALUE | +-----------------+----------------+ | autocommit | ON | | max_connections | 1000 | +-----------------+----------------+ 5032