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Re: Possible bug in query planner when using DISTINCT

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: n(dot)howden(at)eris(dot)qinetiq(dot)com
Cc: pgsql-bugs(at)postgresql(dot)org
Subject: Re: Possible bug in query planner when using DISTINCT
Date: 2002-11-14 19:25:33
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-bugs
Nick Howden <n(dot)howden(at)eris(dot)qinetiq(dot)com> writes:
> Using postgres 7.2.3 the query planner seems to lose interest in using 
> indexes when DISTINCT is used.

I don't think this is a bug.  DISTINCT requires a sort step, so even
though you have a LIMIT, the planner has to plan on the basis of reading
the entire join table.  The nestloop plan loses badly on that basis.

			regards, tom lane

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