Upgrade on mysql get stuck on 2022090900

[gbarat87]

The upgrade step for the version 2022090900 takes forever on mysql with around 800000 rows in the table.

https://github.com/danmarsden/moodle-mod_attendance/blob/MOODLE_401_STABLE/db/upgrade.php#L745

[golenkovm]

Hey @gbarat87 had a look at the patch. It's great you managed to resolve the issue by adding the index. However, it feels like we won't need that index on permanent basis. Also we need it for MySQL db family only. I'd suggest to simplify your patch, add the index before that SQL and drop the index after records are deleted.

[danmarsden]

feels like we're getting pretty hacky if we're adding indexes then removing them within the inline sql (and it's already pretty hacky having different sql for each db type.) - I think I'd prefer to just pull it out of the upgrade script and trigger an adhoc task to perform the clean up or force people to use a CLI script to do the work if they want to.