Re: select distinct and index usage

"David Wilson" <david(dot)t(dot)wilson(at)gmail(dot)com> writes:
> It appears to be doing a sequential scan regardless of the set, as if
> it doesn't believe it can use the index for some reason

More likely, it's getting a cost estimate for the indexscan that's so
bad that it even exceeds the 100000000-unit thumb on the scales that's
inserted by enable_seqscan=off.

You could try setting enable_sort=off also, which'd give you another
100000000 worth of thumb on the scales. And if that doesn't help,
reduce random_page_cost to 1 or even less.

What I think you'll find, though, is that once you do force an indexscan
to be picked it'll be slower. Full-table index scans are typically
worse than seqscan+sort, unintuitive though that may sound.

regards, tom lane