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. 

## 27.12.14.1 Performance Schema persisted_variables Table 

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): 

{ "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" } } } } 

Then persisted_variables  has these contents: 

mysql> SELECT * FROM performance_schema.persisted_variables; +-----------------+----------------+ | VARIABLE_NAME | VARIABLE_VALUE | +-----------------+----------------+ | autocommit | ON | | max_connections | 1000 | +-----------------+----------------+ 

The persisted_variables  table has these columns: 

• VARIABLE_NAME 

The variable name listed in mysqld-auto.cnf . 

• VARIABLE_VALUE 

