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Excerpts from this Manual

22.9.9.1 Event Wait Summary Tables

The Performance Schema maintains tables for collecting current and recent wait events, and aggregates that information in summary tables. Section 22.9.4, “Performance Schema Wait Event Tables” describes the events on which wait summaries are based. See that discussion for information about the content of wait events, the current and recent wait event tables, and how to control wait event collection.

Each event waits summary table has one or more grouping columns to indicate how the table aggregates events. Event names refer to names of event instruments in the setup_instruments table.

  • events_waits_summary_global_by_event_name has an EVENT_NAME column. Each row summarizes events for a given event name. An instrument might be used to create multiple instances of the instrumented object. For example, if there is an instrument for a mutex that is created for each connection, there are as many instances as there are connections. The summary row for the instrument summarizes over all these instances.

  • events_waits_summary_by_instance has EVENT_NAME and OBJECT_INSTANCE_BEGIN columns. Each row summarizes events for a given event name and object. If an instrument is used to create multiple instances, each instance has a unique OBJECT_INSTANCE_BEGIN value, so these instances are summarized separately in this table.

  • events_waits_summary_by_thread_by_event_name has THREAD_ID and EVENT_NAME columns. Each row summarizes events for a given thread and event name.

All event waits summary tables have these summary columns containing aggregated values:

  • COUNT_STAR

    The number of summarized events. This value includes all events, whether timed or nontimed.

  • SUM_TIMER_WAIT

    The total wait time of the summarized timed events. This value is calculated only for timed events because nontimed events have a wait time of NULL. The same is true for the other xxx_TIMER_WAIT values.

  • MIN_TIMER_WAIT

    The minimum wait time of the summarized timed events.

  • AVG_TIMER_WAIT

    The average wait time of the summarized timed events.

  • MAX_TIMER_WAIT

    The maximum wait time of the summarized timed events.

Example wait event summary information:

mysql> SELECT * FROM events_waits_summary_global_by_event_name\G
...
*************************** 6. row ***************************
    EVENT_NAME: wait/synch/mutex/sql/BINARY_LOG::LOCK_index
    COUNT_STAR: 8
SUM_TIMER_WAIT: 2119302
MIN_TIMER_WAIT: 196092
AVG_TIMER_WAIT: 264912
MAX_TIMER_WAIT: 569421
...
*************************** 9. row ***************************
    EVENT_NAME: wait/synch/mutex/sql/hash_filo::lock
    COUNT_STAR: 69
SUM_TIMER_WAIT: 16848828
MIN_TIMER_WAIT: 0
AVG_TIMER_WAIT: 244185
MAX_TIMER_WAIT: 735345
...

TRUNCATE TABLE is permitted for wait summary tables. It resets the summary columns to zero rather than removing rows.


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