MySQL 8.0 Release Notes
MySQL 8.0 Source Code Documentation
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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 themysqld-auto.cnffile that stores persisted global system variable settings. See Section 26.12.13.1, “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 26.12.13.2, “Performance Schema variables_info Table”.
The session variable tables
(session_variables,
variables_by_thread) contain
information only for active sessions, not terminated sessions.
TRUNCATE TABLE is not supported
for Performance Schema system variable tables.
The global_variables and
session_variables tables have these
columns:
VARIABLE_NAMEThe system variable name.
VARIABLE_VALUEThe system variable value. For
global_variables, this column contains the global value. Forsession_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_IDThe thread identifier of the session in which the system variable is defined.
VARIABLE_NAMEThe system variable name.
VARIABLE_VALUEThe session variable value for the session named by the
THREAD_IDcolumn.
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 will miss some rows. In this case, the
Performance_schema_thread_instances_lost
status variable will be greater than zero.