From: Tom Lane <tgl@sss.pgh.pa.us>
To: Vlad Arkhipov <arhipov@dc.baikal.ru>
cc: pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: Why don't use index on x when ORDER BY x, y?
In-reply-to: <5473103A.5040004@dc.baikal.ru>
References: <5473103A.5040004@dc.baikal.ru>
Comments: In-reply-to Vlad Arkhipov <arhipov@dc.baikal.ru>
 message dated "Mon, 24 Nov 2014 19:02:18 +0800"
Date: Mon, 24 Nov 2014 10:38:46 -0500
Message-ID: <23348.1416843526@sss.pgh.pa.us>
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Sender: pgsql-performance-owner@postgresql.org

Vlad Arkhipov <arhipov@dc.baikal.ru> writes:
> I wonder why Postgres does not use index in the query below?

Because it's useless: you'd still have to do a sort, and an indexscan
is going to be a slower source of data for the sort than a seqscan.

There's been some experimentation of late with a "partial sort" capability
that could take advantage of partially-ordered input, which might make
this kind of thing interesting after all.  But it's not committed and
might never be: it's far from clear that it'd be a win in many cases.

			regards, tom lane


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