The "keyset"-style pagination that Tildes uses for topic listings uses
WHERE and ORDER BY clauses that involve multiple columns to keep a
deterministic ordering even when the values in the main sort column are
equal. For example, when sorting by number of votes, you're actually
ordering by num_votes DESC, topic_id DESC. The previous single-column
indexes were a little inefficient for this and couldn't always be used
well.
This commit extends all of the relevant indexes to composite ones that
contain topic_id as well, and drops all of the original ones. This
should be more efficient, and should probably be done to indexes on the
comments table as well.
Deimos
47657c988e