25.8 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:
Row# EVENT_NAME TIMER_START TIMER_END ---- ---------- ----------- --------- 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.