26.1 Performance Schema Quick Start

This section briefly introduces the Performance Schema with examples that show how to use it. For additional examples, see Section 26.19, “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:

[mysqld]
performance_schema=ON

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.

The Performance Schema is implemented as a storage engine, so you will see it listed in the output from the INFORMATION_SCHEMA.ENGINES table or the SHOW ENGINES statement:

The PERFORMANCE_SCHEMA storage engine operates on tables in the performance_schema database. You can make performance_schema the default database so that references to its tables need not be qualified with the database name:

Performance Schema tables are stored in the performance_schema database. Information about the structure of this database and its tables can be obtained, as for any other database, by selecting from the INFORMATION_SCHEMA database or by using SHOW statements. For example, use either of these statements to see what Performance Schema tables exist:

The number of Performance Schema tables increases over time as implementation of additional instrumentation proceeds.

The name of the performance_schema database is lowercase, as are the names of tables within it. Queries should specify the names in lowercase.

To see the structure of individual tables, use SHOW CREATE TABLE:

Table structure is also available by selecting from tables such as INFORMATION_SCHEMA.COLUMNS or by using statements such as SHOW COLUMNS.

Tables in the performance_schema database can be grouped according to the type of information in them: Current events, event histories and summaries, object instances, and setup (configuration) information. The following examples illustrate a few uses for these tables. For detailed information about the tables in each group, see Section 26.12, “Performance Schema Table Descriptions”.

Initially, not all instruments and consumers are enabled, so the performance schema does not collect all events. To turn all of these on and enable event timing, execute two statements (the row counts may differ depending on MySQL version):

To see what the server is doing at the moment, examine the events_waits_current table. It contains one row per thread showing each thread's most recent monitored event:

This event indicates that thread 0 was waiting for 86,526 picoseconds to acquire a lock on THR_LOCK::mutex, a mutex in the mysys subsystem. The first few columns provide the following information:

The history tables contain the same kind of rows as the current-events table but have more rows and show what the server has been doing “recently” rather than “currently.” The events_waits_history and events_waits_history_long tables contain the most recent 10 events per thread and most recent 10,000 events, respectively. For example, to see information for recent events produced by thread 13, do this:

As new events are added to a history table, older events are discarded if the table is full.

Summary tables provide aggregated information for all events over time. The tables in this group summarize event data in different ways. To see which instruments have been executed the most times or have taken the most wait time, sort the events_waits_summary_global_by_event_name table on the COUNT_STAR or SUM_TIMER_WAIT column, which correspond to a COUNT(*) or SUM(TIMER_WAIT) value, respectively, calculated over all events:

These results show that the THR_LOCK_malloc mutex is “hot,” both in terms of how often it is used and amount of time that threads wait attempting to acquire it.

Instance tables document what types of objects are instrumented. An instrumented object, when used by the server, produces an event. These tables provide event names and explanatory notes or status information. For example, the file_instances table lists instances of instruments for file I/O operations and their associated files:

Setup tables are used to configure and display monitoring characteristics. For example, setup_instruments lists the set of instruments for which events can be collected and shows which of them are enabled:

To understand how to interpret instrument names, see Section 26.6, “Performance Schema Instrument Naming Conventions”.

To control whether events are collected for an instrument, set its ENABLED value to YES or NO. For example:

The Performance Schema uses collected events to update tables in the performance_schema database, which act as “consumers” of event information. The setup_consumers table lists the available consumers and which are enabled:

To control whether the Performance Schema maintains a consumer as a destination for event information, set its ENABLED value.

For more information about the setup tables and how to use them to control event collection, see Section 26.4.2, “Performance Schema Event Filtering”.

There are some miscellaneous tables that do not fall into any of the previous groups. For example, performance_timers lists the available event timers and their characteristics. For information about timers, see Section 26.4.1, “Performance Schema Event Timing”.