Joseph Shraibman <jks@selectacast.net> writes:
> This raises some other questions. Why can't postgres get the count(*)
> from the index? Why doesn't it predict the correct number of rows in
> the planner? (25 estimated vs 16 actual).
The name of the game here is to make a plan *without* actually going
out and expending large amounts of time to find out the true state of
affairs; by the time you know for sure, you've already done the query.
We have to do a certain amount of guessing, otherwise the planner will
be a net drag on performance. Accordingly, the estimates will never be
perfectly accurate.
regards, tom lane