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Re: distinct vs group by

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com>
Cc: pgsql-novice(at)postgresql(dot)org
Subject: Re: distinct vs group by
Date: 2005-01-29 22:14:07
Message-ID: 25772.1107036847@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-novice
Ron Mayer <rm_pg(at)cheapcomplexdevices(dot)com> writes:
> Should the expressions
>   select distinct x from t
> and 
>   select          x from t group by x
> have the same effect?  

Offhand I think they are theoretically equivalent.

> It seems the optimizer sometimes chooses different plans for those
> expressions.   Could the select distinct have used the slightly
> faster hash aggregate?

This is partly historical (the DISTINCT code hasn't been rewritten in a
long time) and partly intentional --- you can choose one phrasing or the
other to control what plan you get.

			regards, tom lane

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