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26.8 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.
Document created the 26/06/2006, last modified the 26/10/2018
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