If you see the “Waiting for table metadata lock” error, you may be wondering what is the best course of action to prevent it in the future. I have briefly discussed troubleshooting metadata locks before, however, that post more illustrated how easy it is to encounter the “Waiting for table metadata lock” error in both MyISAM and InnoDB. It provides simple examples for reproducing using both storage engines, so one could hopefully more easily identify where the locks are stemming from in their particular case.
Pin-pointing which transaction holds the locks is a different story. There are feature requests filed in MySQL and MariaDB bugs databases to track this information, but they are recent, and no ETA is scheduled for either yet, as far as I know:[Read more]