Missing index for queries run during upgrade #2710

During upgrade, some SQL DELETE requests may be run on the activity table that would miss an index to be run quickly.
The request have a WHERE clause lie this one:

WHERE affecteduser <> ...
  AND object_type = 'calendar'
  AND object_id = ...
  AND subjectparams LIKE '%...%'

Creating a new index like this

ALTER TABLE nextcloud_db.oc_activity
ADD INDEX activity_object_cleanup_idx (object_type, object_id),
ALGORITHM=INPLACE,
LOCK=NONE;

This changed the execution plan from a full table scan to an index query.
Doing that solved the slow upgrade process.