Replace use of SHOW TABLES with DESCRIBE.
#834
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… old version cases.
…ue after upgraded to v7.0.
`SHOW TABLES` performs very poorly on large databases. `DESCRIBE` is a low-overhead replacement.
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I recently updated from 6.x to 7.0.1 on a very large WP Multisite installation. MySQL ground to a halt shortly afterward, and after investigation we determined that the enormous number of
SHOW TABLESqueries coming fromlitespeed-cachewere the culprit.SHOW TABLESrequires touchingINFORMATION_SCHEMA, which on very large databases means temp sort and other operations that require enormous resources. See https://bugs.mysql.com/bug.php?id=60961, https://www.percona.com/blog/solving-information_schema-slowness/, https://forums.mysql.com/read.php?24,706979,706979Every instance of
SHOW TABLESin this plugin uses a hardcoded value (no wildcards) and is only check for existence. As such, we can useDESCRIBE(or alternativelySHOW CREATE TABLE), which only loads information about the queried table and doesn't require loading all table info into memory.I'm running this patch on my client site and the performance problems have gone away.
Thanks in advance for considering.