Queries with the combination of account_id, id, and visibility can be
categorized in three types:
1. Querying for public and unlisted to enumerate statuses visible to
anyone.
2. Querying for public, unlisted, and private to enumerate statuses
visible to follower.
3. Querying for direct to enumerate own direct statuses.
1 and 2 is covered by the index with condition 'visibility IN (0, 1, 2)'.
It would bring better performance in case that there are many direct
statuses.
The index with condition 'visibility = 3' is just for 3. It would be much
faster to query direct statuses thanks to this query.
The total size of those two indexes are expected to be smaller than the
deleted one because they are partial and does not have to cover all the
table.
Akihiko Odaki