Re: Shouldn't the planner have a higher cost for reverse index scans?

Lists <lists(at)on-track(dot)ca> 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