The Performance Schema avoids using mutexes to collect or produce data, so there are no guarantees of consistency and results can sometimes be incorrect. Event values in 'performance_schema' tables are nondeterministic and nonrepeatable.
If you save event information in another table, you should not assume that the original events are still available later. For example, if you select events from a 'performance_schema' table into a temporary table, intending to join that table with the original table later, there might be no matches.
*note 'mysqldump': mysqldump. and 'BACKUP DATABASE' ignore tables in the 'performance_schema' database.
Tables in the 'performance_schema' database cannot be locked with 'LOCK TABLES', except the 'setup_XXX' tables.
Tables in the 'performance_schema' database cannot be indexed.
Results for queries that refer to tables in the 'performance_schema' database are not saved in the query cache.
Tables in the 'performance_schema' database are not replicated.
The Performance Schema is not available in 'libmysqld', the embedded server.
The types of timers might vary per platform. The *note 'performance_timers': performance-schema-performance-timers-table. table shows which event timers are available. If the values in this table for a given timer name are 'NULL', that timer is not supported on your platform.
Instruments that apply to storage engines might not be implemented for all storage engines. Instrumentation of each third-party engine is the responsibility of the engine maintainer.
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26 MySQL sys Schema *******************
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sys-schema-reference:: sys Schema Object Reference
MySQL 5.7 includes the note 'sys': sys-schema. schema, a set of objects that helps DBAs and developers interpret data collected by the Performance Schema. note 'sys': sys-schema. schema objects can be used for typical tuning and diagnosis use cases. Objects in this schema include:
Views that summarize Performance Schema data into more easily understandable form.
Stored procedures that perform operations such as Performance Schema configuration and generating diagnostic reports.
Stored functions that query Performance Schema configuration and provide formatting services.
For new installations, the note 'sys': sys-schema. schema is installed by default during data directory initialization if you use note 'mysqld': mysqld. with the '--initialize' or '--initialize-insecure' option. You can drop the *note 'sys': sys-schema. schema manually after initialization if it is unneeded.
For upgrades, note 'mysql_upgrade': mysql-upgrade. installs the note 'sys': sys-schema. schema if it is not installed, and upgrades it to the current version otherwise. To permit this behavior to be suppressed, *note 'mysql_upgrade': mysql-upgrade. has a '--skip-sys-schema' option.
note 'mysql_upgrade': mysql-upgrade. returns an error if a note 'sys': sys-schema. schema exists but has no note 'version': sys-version. view, on the assumption that absence of this view indicates a user-created 'sys' schema. To upgrade in this case, remove or rename the existing note 'sys': sys-schema. schema first.
*note 'sys': sys-schema. schema objects have a 'DEFINER' of ''mysql.sys'@'localhost''. Use of the dedicated 'mysql.sys' account avoids problems that occur if a DBA renames or removes the 'root' account.
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