| From: | Josh Berkus <josh(at)agliodbs(dot)com> |
|---|---|
| To: | Ryan Bradetich <rbradetich(at)uswest(dot)net> |
| Cc: | Oleg Lebedev <oleg(dot)lebedev(at)waterford(dot)org>, "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: slow query |
| Date: | 2003-02-23 21:47:58 |
| Message-ID: | 200302231347.58960.josh@agliodbs.com |
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| Lists: | pgsql-performance |
Ryan,
> > The above should use an EXISTS clause, not IN, unless you are absolutely
> > sure that the subquery will never return more than 12 rows.
>
> I am assuming you said this because EXISTS is faster for > 12 rows?
> Interesting :)
That's my rule of thumb, *NOT* any kind of relational-calculus-based truth.
Basically, one should only use IN for a subquery when one is *absolutely* sure
that the subquery will only return a handful of records, *and* the subquery
doesn't have to do an complex work like aggregating or custom function
evaluation.
You're safer using EXISTS for all subqueries, really.
--
Josh Berkus
Aglio Database Solutions
San Francisco
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