Re: Why does the query planner use two full indexes, when a dedicated partial index exists?

Richard Neill <rn214(at)richardneill(dot)org> writes:
> The problem is, when I now run my query, the planner ignores the
> dedicated index "tbl_tracker_performance_1_idx", and instead uses both
> of the full indexes... resulting in a much much slower query (9ms vs
> 0.08ms).

> A psql session is below. This shows that, if I force the planner to use
> the partial index, by dropping the others, then it's fast. But as soon
> as I put the full indexes back (which I need for other queries), the
> query planner chooses them instead, and is slow.

[ experiments with a similar test case ... ] I think the reason why the
planner is overestimating the cost of using the partial index is that
9.1 and earlier fail to account for the partial-index predicate when
estimating the number of index rows that will be visited. Because the
partial-index predicate is so highly selective in this case, that
results in a significant overestimate of how much of the index will be
traversed.

We fixed this for 9.2 in
http://git.postgresql.org/gitweb/?p=postgresql.git&a=commitdiff&h=21a39de5809cd3050a37d2554323cc1d0cbeed9d
but did not want to risk back-patching such a behavioral change. If
you're stuck on 9.1 you might want to think about applying that as a
local patch though.

(BTW, the "fudge factor" change in that patch has been criticized
recently; we've changed it again already for 9.3 and might choose to
back-patch that into 9.2.3. But it's the rest of it that you care about
anyway.)

regards, tom lane