To: "Richard Huxton" <dev@archonet.com>
Cc: "Paul Lambert" <paul.lambert@reynolds.com.au>,
	<pgsql-performance@postgresql.org>
Subject: Re: Optimising a query
In-Reply-To: <4768DAC0.4010304@archonet.com> (Richard Huxton's message of
	"Wed\, 19 Dec 2007 08\:48\:00 +0000")
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References: <4768B4D1.1090702@reynolds.com.au>
	<4768BE4B.9080308@reynolds.com.au> <4768DAC0.4010304@archonet.com>
From: Gregory Stark <stark@enterprisedb.com>
Organization: EnterpriseDB
Date: Wed, 19 Dec 2007 09:50:29 +0000
Message-ID: <87ve6vhv2y.fsf@oxford.xeocode.com>
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> Also, try replacing the DISTINCT with GROUP BY. The code path for DISTINCT
> unfortunately needs a bit of cleaning up and isn't exactly equivalent to GROUP
> BY. In particular it doesn't support hash aggregates which, if your work_mem
> is large enough, might work for you here.

Sorry, strike that last suggestion. I was looking at the plan and forgot that
the query had DISTINCT ON. It is possible to replace DISTINCT ON with GROUP BY
but it's not going to be faster than the DISTINCT ON case since you'll need
the sort anyways.

Actually it's possible to do without the sort if you write some fancy
aggregate functions but for this large a query that's going to be awfully
complex.

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Get trained by Bruce Momjian - ask me about EnterpriseDB's PostgreSQL training!