I recently wrote a post on tracking metadata locks (MDL) in MySQL 5.7, and I wanted to take a moment to expand on it by explaining a couple of the associated variables in more detail.
First off, once you have enabled the performance_schema *and* the metadata lock instrumentation, you can verify it with:
mysql> SELECT * FROM performance_schema.setup_instruments
-> WHERE NAME = 'wait/lock/metadata/sql/mdl';
+----------------------------+---------+-------+
| NAME | ENABLED | TIMED |
+----------------------------+---------+-------+
| wait/lock/metadata/sql/mdl | YES | YES |
+----------------------------+---------+-------+
“ENABLED” will report “YES” if it is enabled properly, and “NO” if not.
“TIMED” (referring to event timing) reports “YES” if it was enabled via the my.cnf or my.ini file, and it reports …
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