| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Joseph Shraibman <jks(at)selectacast(dot)net> |
| Cc: | pgsql-general(at)postgresql(dot)org |
| Subject: | Re: Weird indices |
| Date: | 2001-02-20 02:34:34 |
| Message-ID: | 10763.982636474@sss.pgh.pa.us |
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| Lists: | pgsql-general |
Joseph Shraibman <jks(at)selectacast(dot)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
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