| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Bruno Wolff III <bruno(at)wolff(dot)to> |
| Cc: | Arjen van der Meijden <acmmailing(at)vulcanus(dot)its(dot)tudelft(dot)nl>, pgsql-hackers(at)postgreSQL(dot)org, pgsql-performance(at)postgreSQL(dot)org |
| Subject: | Re: Recognizing range constraints (was Re: Plan for relatively simple query seems to be very inefficient) |
| Date: | 2005-04-07 14:20:24 |
| Message-ID: | 16907.1112883624@sss.pgh.pa.us |
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| Lists: | pgsql-hackers pgsql-performance |
Bruno Wolff III <bruno(at)wolff(dot)to> writes:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Can anyone suggest a more general rule?
> I think it makes sense to guess that a smaller fraction of the rows will
> be returned when a column value is bounded above and below than if it
> is only bounded on one side, even if the bounds aren't fixed. You can
> certainly be wrong.
Yeah, the whole thing is only a heuristic anyway. I've been coming
around to the view that relation membership shouldn't matter, because
of cases like
WHERE a.x > b.y AND a.x < 42
which surely should be taken as a range constraint.
regards, tom lane
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