Performance Schema Quick Start The Performance Schema is intended to provide access to useful information about server execution while having minimal impact on server performance. The implementation follows these design goals: •Activating the Performance Schema causes no changes in server behavior. For example, it does not cause thread scheduling to change, and it does not cause query execution plans (as shown by EXPLAIN ) to change. •Server monitoring occurs continuously and unobtrusively with very little overhead. Activating the Performance Schema does not make the server unusable. •The parser is unchanged. There are no new keywords or statements. •Execution of server code proceeds normally even if the Performance Schema fails internally. •When there is a choice between performing processing during event collection initially or during event retrieval later, priority is given to making collection faster. This is because collection is ongoing whereas retrieval is on demand and might never happen at all. •Most Performance Schema tables have indexes, which gives the optimizer access to execution plans other than full table scans. For more information, see Section 8.2.4, “Optimizing Performance Schema Queries” . •It is easy to add new instrumentation points. •Instrumentation is versioned. If the instrumentation implementation changes, previously instrumented code continues to work. This benefits developers of third-party plugins because it is not necessary to upgrade each plugin to stay synchronized with the latest Performance Schema changes.  The MySQL  sys  schema is a set of objects that provides convenient access to data collected by the Performance Schema. The  sys  schema is installed by default. For usage instructions, see Chapter 28,  MySQL sys Schema . This section briefly introduces the Performance Schema with examples that show how to use it. For additional examples, see Section 27.19, “Using the Performance Schema to Diagnose Problems” . The Performance Schema is enabled by default. To enable or disable it explicitly, start the server with the performance_schema  variable set to an appropriate value. For example, use these lines in the server my.cnf  file: When the server starts, it sees  performance_schema  and attempts to initialize the Performance Schema. To verify successful initialization, use this statement: A value of  ON  means that the Performance Schema initialized successfully and is ready for use. A value of OFF  means that some error occurred. Check the server error log for information about what went wrong. 4895 mysql>  SHOW VARIABLES LIKE ’performance_schema’; 27.1 Performance Schema Quick Start [mysqld] performance_schema=ON +--------------------+-------+ | Variable_name | Value | +--------------------+-------+ | performance_schema | ON | +--------------------+-------+ Note