To: <depesz@depesz.com>
Cc: "JS Ubei" <jsubei@yahoo.fr>,  <pgsql-performance@postgresql.org>
Subject: Re: optimize query with a maximum(date) extraction
In-Reply-To: <20070905132250.GA30304@depesz.com> (hubert depesz lubaczewski's
	message of "Wed\, 5 Sep 2007 15\:22\:50 +0200")
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References: <302443.65402.qm@web26209.mail.ukl.yahoo.com>
	<87abs1qrhu.fsf@oxford.xeocode.com>
	<20070905132250.GA30304@depesz.com>
From: Gregory Stark <stark@enterprisedb.com>
Organization: EnterpriseDB
Date: Wed, 05 Sep 2007 15:14:00 +0100
Message-ID: <871wddqjx3.fsf@oxford.xeocode.com>
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"hubert depesz lubaczewski" <depesz@depesz.com> writes:

> 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.

Cute. I didn't think of that, thanks

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com