Date: Wed, 5 Sep 2007 15:22:50 +0200
From: hubert depesz lubaczewski <depesz@depesz.com>
To: Gregory Stark <stark@enterprisedb.com>
Cc: JS Ubei <jsubei@yahoo.fr>, pgsql-performance@postgresql.org
Subject: Re: optimize query with a maximum(date) extraction
Message-ID: <20070905132250.GA30304@depesz.com>
Reply-To: depesz@depesz.com
References: <302443.65402.qm@web26209.mail.ukl.yahoo.com>
	<87abs1qrhu.fsf@oxford.xeocode.com>
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On Wed, Sep 05, 2007 at 12:30:21PM +0100, Gregory Stark wrote:
> SELECT DISTINCT ON (id) id, the_date AS min_date FROM my_table ORDER BY id, the_date ASC
> SELECT DISTINCT ON (id) id, the_date AS max_date FROM my_table ORDER BY id, the_date DESC
> I think the first of these can actually use your index but the latter can't
> unless you create one for it specifically (which is not so easy -- it'll be
> easier in 8.3 though). Worse, I'm not really sure it'll be any faster than the
> query you already have.

it's easy to fix the second query (fix to use index) - just change order
by to:
order by id desc, the_date desc.

depesz

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