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MySQL 8.4 Reference Manual  /  MySQL Performance Schema  /  Performance Schema Atom and Molecule Events

29.8 Performance Schema Atom and Molecule Events

For a table I/O event, there are usually two rows in events_waits_current, not one. For example, a row fetch might result in rows like this:

---- ----------                 ----------- ---------
   1 wait/io/file/myisam/dfile        10001 10002
   2 wait/io/table/sql/handler        10000 NULL

The row fetch causes a file read. In the example, the table I/O fetch event started before the file I/O event but has not finished (its TIMER_END value is NULL). The file I/O event is nested within the table I/O event.

This occurs because, unlike other atomic wait events such as for mutexes or file I/O, table I/O events are molecular and include (overlap with) other events. In events_waits_current, the table I/O event usually has two rows:

  • One row for the most recent table I/O wait event

  • One row for the most recent wait event of any kind

Usually, but not always, the of any kind wait event differs from the table I/O event. As each subsidiary event completes, it disappears from events_waits_current. At this point, and until the next subsidiary event begins, the table I/O wait is also the most recent wait of any kind.