I’m sure you’ve seen it  you kick off an ALTER and you get the dreaded “waiting on metadata lock”.  In many cases, this is expected if you are actively working on the table. However, I recently had a case with a client where the table being altered was rarely updated and very small (<100 [&]

https://www.percona.com/blog/2019/07/02/alters-foreign-keys-and-metadata-locks-oh-my/