25.20 Migrating to Performance Schema System and Status Variable Tables

The INFORMATION_SCHEMA has tables that contain system and status variable information (see Section 24.3.11, “The INFORMATION_SCHEMA GLOBAL_VARIABLES and SESSION_VARIABLES Tables”, and Section 24.3.10, “The INFORMATION_SCHEMA GLOBAL_STATUS and SESSION_STATUS Tables”). The Performance Schema also contains system and status variable tables (see Section 25.12.13, “Performance Schema System Variable Tables”, and Section 25.12.14, “Performance Schema Status Variable Tables”). The Performance Schema tables are intended to replace the INFORMATION_SCHEMA tables, which are deprecated as of MySQL 5.7.6 and are removed in MySQL 8.0.

This section describes the intended migration path away from the INFORMATION_SCHEMA system and status variable tables to the corresponding Performance Schema tables. Application developers should use this information as guidance regarding the changes required to access system and status variables in MySQL 5.7.6 and up as the INFORMATION_SCHEMA tables become deprecated and eventually are removed.

MySQL 5.6

In MySQL 5.6, system and status variable information is available from these SHOW statements:

SHOW VARIABLES
SHOW STATUS

And from these INFORMATION_SCHEMA tables:

INFORMATION_SCHEMA.GLOBAL_VARIABLES
INFORMATION_SCHEMA.SESSION_VARIABLES

INFORMATION_SCHEMA.GLOBAL_STATUS
INFORMATION_SCHEMA.SESSION_STATUS

MySQL 5.7

As of MySQL 5.7.6, the Performance Schema includes these tables as new sources of system and status variable information:

performance_schema.global_variables
performance_schema.session_variables
performance_schema.variables_by_thread

performance_schema.global_status
performance_schema.session_status
performance_schema.status_by_thread
performance_schema.status_by_account
performance_schema.status_by_host
performance_schema.status_by_user

MySQL 5.7.6 also adds a show_compatibility_56 system variable to control how the server makes system and status variable information available.

When show_compatibility_56 is ON, compatibility with MySQL 5.6 is enabled. The older system and status variable sources (SHOW statements, INFORMATION_SCHEMA tables) are available with semantics identical to MySQL 5.6. Applications should run as is, with no code changes, and should see the same variable names and values as in MySQL 5.6. Warnings occur under these circumstances:

A deprecation warning is raised when selecting from the INFORMATION_SCHEMA tables.
In MySQL 5.7.6 and 5.7.7, a deprecation warning is raised when using a WHERE clause with the SHOW statements. This behavior does not occur as of MySQL 5.7.8.

When show_compatibility_56 is OFF, compatibility with MySQL 5.6 is disabled and several changes result. Applications must be revised as follows to run properly:

Selecting from the INFORMATION_SCHEMA tables produces an error. Applications that access the INFORMATION_SCHEMA tables should be revised to use the corresponding Performance Schema tables instead.
Before MySQL 5.7.9, selecting from the INFORMATION_SCHEMA tables produces an empty result set plus a deprecation warning. This was not sufficient notice to signal the need to migrate to the corresponding Performance Schema system and status variable tables for the case that show_compatibility_56=OFF. Producing an error in MySQL 5.7.9 and higher makes it more evident that an application is operating under conditions that require modification, as well as where the problem lies.
In MySQL 5.7.6 and 5.7.7, the Performance Schema session_variables and session_status tables do not fully reflect all variable values in effect for the current session; they include no rows for global variables that have no session counterpart. This is corrected in MySQL 5.7.8.
Output for the SHOW statements is produced using the underlying Performance Schema tables. Applications written to use these statements can still use them, but it is best to use MySQL 5.7.8 or higher. In MySQL 5.7.6 and 5.7.7, the results may differ:
- SHOW [SESSION] VARIABLES output does not include global variables that have no session counterpart.
- Using a WHERE clause with the SHOW statements produces an error.
These Slave_xxx status variables become unavailable through SHOW STATUS:
```
Slave_heartbeat_period
Slave_last_heartbeat
Slave_received_heartbeats
Slave_retried_transactions
Slave_running
```
Applications that use these status variables should be revised to obtain this information using the replication-related Performance Schema tables. For details, see Effect of show_compatibility_56 on Slave Status Variables.
The Performance Schema does not collect statistics for Com_xxx status variables in the status variable tables. To obtain global and per-session statement execution counts, use the events_statements_summary_global_by_event_name and events_statements_summary_by_thread_by_event_name tables, respectively.

Migration and Privileges

Initially, with the introduction of Performance Schema system and status variable tables in MySQL 5.7.6, access to those tables required the SELECT privilege, just as for other Performance Schema tables. However, this had the consequence that when show_compatibility_56=OFF, the SHOW VARIABLES and SHOW STATUS statements also required the SELECT privilege: With compatibility disabled, output for those statements was taken from the Performance Schema global_variables, session_variables, global_status, and session_status tables.

As of MySQL 5.7.9, those Performance Schema tables are world readable and accessible without the SELECT privilege. Consequently, SHOW VARIABLES and SHOW STATUS do not require privileges on the underlying Performance Schema tables from which their output is produced when show_compatibility_56=OFF.

Beyond MySQL 5.7

In a MySQL 8.0, the INFORMATION_SCHEMA variable tables and the show_compatibility_56 system variable are removed, and output from the SHOW statements is always based on the underlying Performance Schema tables.

Applications that have been revised to work in MySQL 5.7 when show_compatibility_56=OFF should work without further changes, except that it is not possible to test or set show_compatibility_56 because it does not exist.