Message-ID: <4B85B259.4030900@archonet.com>
Date: Wed, 24 Feb 2010 23:12:25 +0000
From: Richard Huxton <dev@archonet.com>
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To: Dave Crooke <dcrooke@gmail.com>
CC: Garrett Murphy <gmurphy@lawlogix.com>,
	pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: Extracting superlatives - SQL design philosophy
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On 24/02/10 22:47, Dave Crooke wrote:
> I'd imagine it would be possible to have a query planner optimization
> that would convert Garrett's DISTINCT ON syntax to do what I was
> trying to, by realizing that DISTINCT ON X ... ORDER BY Y DESC is
> going to return the the one row for each X which has the highest value
> of Y, and so use a MAX-structured accumulation instead of a sort.

Why is there only one row? For city temperatures, that seems unlikely.

In the event of more than one row does your algorithm give repeatable 
results?

-- 
   Richard Huxton
   Archonet Ltd