First reported in https://meta.discourse.org/t/-/202482/19
There are two optimizations being applied here:
1. Fetch a user's group ids in a seperate query instead of including it
as a sub-query. When I tried a subquery, the query plan becomes very
inefficient.
1. Join against the `topic_allowed_users` and `topic_allowed_groups`
table instead of doing an IN against a subquery where we UNION the
`topic_id`s from the two tables. From my profiling, this enables PG to
do a backwards index scan on the `index_topics_on_timestamps_private`
index.
This commit fixes a bug where listing all messages was incorrectly
excluding topics if a topic has been archived by a group even if the
user did not belong to the group.
This commit also fixes another bug where dismissing private messages
selectively was subjected to the default limit of 30.
