Re: Query optimizer plans with very small selectivity estimates

Matthew Bellew <matthewb(at)labkey(dot)com> writes:
> I made have several users encounter performance problems, which all
> seem to come down to this problem: multiplying selectivity estimates can
> cause tuple estimates to grow very small very quickly, once the estimator
> gets to 1 row, the planner may choose plans that are very good ONLY WHEN
> there is exactly 1 row (maybe even O(N^large)). Unfortunately, these may
> be the worst plans if the estimate is even slightly off (even just
> returning 2 or 3 rows versus 1).

Yeah, this is a well-known problem. There has been prior discussion along
the same lines as you mention (only believe 1-row estimates when it's
provably true that there's at most one row), but it hasn't looked like an
easy change. See the pgsql-hackers archives for previous threads.

regards, tom lane