| From: | "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov> |
|---|---|
| To: | "Scott Marlowe" <scott(dot)marlowe(at)gmail(dot)com>, "Oliveiros d'Azevedo Cristina" <oliveiros(dot)cristina(at)marktest(dot)pt>, "Jonathan Hoover" <jhoover(at)yahoo-inc(dot)com> |
| Cc: | <pgsql-admin(at)postgresql(dot)org> |
| Subject: | Re: Get Postgres to use multiple proc cores? |
| Date: | 2010-11-05 16:08:40 |
| Message-ID: | 4CD3E5B8020000250003735A@gw.wicourts.gov |
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| Lists: | pgsql-admin |
"Oliveiros d'Azevedo Cristina" <oliveiros(dot)cristina(at)marktest(dot)pt>
wrote:
> What is faster? A JOIN or a WHERE a IN (SELECT ... )
On 8.4 and later an EXISTS or NOT EXISTS will use semi-join or
anti-join (respectively). These should usually be much faster than
the IN (SELECT DISTINCT ... ) technique. Be careful of the
difference in semantics between NOT EXISTS and NOT IN if any of the
columns involved in the matching can be null -- they are *not* the
same thing.
-Kevin
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