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Re: DISTINCT -> GROUP BY

From: "Pavel Stehule" <pavel(dot)stehule(at)gmail(dot)com>
To: "David Fetter" <david(at)fetter(dot)org>
Cc: "PG Hackers" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: DISTINCT -> GROUP BY
Date: 2008-06-03 13:36:44
Message-ID: 162867790806030636p17d6f9b4g8f65f3ed42dbf969@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Hello David

http://www.postgresql.org/docs/faqs.TODO.html

Consider using hash buckets to do DISTINCT, rather than sorting
This would be beneficial when there are few distinct values. This is
already used by GROUP BY.

Regards
Pavel Stehule

2008/6/3 David Fetter <david(at)fetter(dot)org>:
> Folks,
>
> I've noticed that queries of the form
>
> SELECT DISTNCT foo, bar, baz
> FROM quux
> WHERE ...
>
> perform significantly worse than the equivalent using GROUP BY.
>
> SELECT foo, bar, baz
> FROM quux
> WHERE ...
> GROUP BY foo, bar, baz
>
> Where would I start looking in order to make them actually equivalent
> from the planner's point of view?  Also, would back-patching this make
> sense?  It doesn't change any APIs, but it does make some queries go
> faster.
>
> Cheers,
> David.
> --
> David Fetter <david(at)fetter(dot)org> http://fetter.org/
> Phone: +1 415 235 3778  AIM: dfetter666  Yahoo!: dfetter
> Skype: davidfetter      XMPP: david(dot)fetter(at)gmail(dot)com
>
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