Re: Weird indices

On Mon, 19 Feb 2001, Joseph Shraibman wrote:

> > Of course, if the 10113-match estimate is wildly off (as it was in this
> > case), then the wrong plan may be chosen.  But it IS NOT CORRECT to
> > suppose that indexscans always beat seqscans.  The planner's job would
> > be a lot easier if that were true.
>
> Can't postgres do the index lookup first and find out there are only a
> few tuples that might match?

Well, theoretically the estimate is supposed to match reality.  There are
still some cases where there isn't enough information kept to allow that
to be true (the case where there is a single very common non-NULL value is
one such case).