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?
Date: Mon, 24 Nov 2014 10:38:46 -0500
Message-ID: <23348.1416843526@sss.pgh.pa.us> (raw)
In-Reply-To: <5473103A.5040004@dc.baikal.ru>
References: <5473103A.5040004@dc.baikal.ru>
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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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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, arhipov@dc.baikal.ru
Subject: Re: Why don't use index on x when ORDER BY x, y?
In-Reply-To: <23348.1416843526@sss.pgh.pa.us>
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