Re: aggregate on zero rows slow?

On Thu, 2 May 2002, Patrick Welche wrote:

> On Wed, May 01, 2002 at 10:21:48PM -0400, Tom Lane wrote:
> > Patrick Welche <prlw1(at)newn(dot)cam(dot)ac(dot)uk> writes:
> > > ... the select appears to take much longer to complete if the WHERE clause
> > > matches zero rows, than if it matches some rows.
> >
> > Doesn't make any sense to me. Are you sure the same plan is being
> > chosen in both cases? If so, could you post a complete example?
>
> Yes, same plan both cases. Thanks for the answer: as it doesn't make any
> sense to anyone, it must mean there is some corruption somewhere as a result
> of Monday morning's powercut => I'll look elsewhere.

Could it possibly be that, since it can't find any rows, it's fully
exhausting the indexes looking for the values it wants? I mean, a match
would presumably be found in the indexes before hitting the end, but a
non-match wouldn't. If the index is big enough, I could see a query
that returned no results taking longer than one which does.

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