Lists <lists@xxxxxxxxxxx> writes: > I already had a separate index on uid > CREATE INDEX idx_user_comments_uid > ON user_comments > USING btree > (uid); > Under the circumstances, shouldn't a bitmap of those 2 indexes be far > faster than using just the date index (compared to the old plan, not the > new composite index). Why would the planner not choose that plan? It wouldn't produce sorted output; you'd have to read all the rows with uid 1 and then sort them to find the lowest [highest] comment_date. I'm sure the planner did consider that, but guessed that the other way would win on average. The fact that you have lots of rows with uid=1 would tend to push its cost estimates in that direction. Unfortunately it doesn't have any clue that the rows with uid=1 are concentrated in older comment_dates, making the approach a loser for the highest-date flavor of the problem. regards, tom lane -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
