"John Surcombe" <John.Surcombe@digimap.gg> writes:
> When we 'EXPLAIN' this query, PostgreSQL says it is using the index
> idx_receiveddatetime. The way the application is designed means that in
> virtually all cases the query will have to scan a very long way into
> idx_receiveddatetime to find the first record where userid = 311369000.
> If however we delete the idx_receiveddatetime index, the query uses the
> idx_userid_receiveddatetime index, and the query only takes a few
> milliseconds.
That's just bizarre ... it knows the index is applicable, and the cost
estimates clearly favor the better index, so why did it pick the worse
one?
I tried to duplicate this locally, without success, so there's some
contributing factor you've neglected to mention. Can you put together a
self-contained test case that acts like this?
regards, tom lane