Re: Why don't use index on x when ORDER BY x, y?

От: Tom Lane
Тема: Re: Why don't use index on x when ORDER BY x, y?
Дата: ,
Msg-id: 23348.1416843526@sss.pgh.pa.us
(см: обсуждение, исходный текст)
Ответ на: Why don't use index on x when ORDER BY x, y?  (Vlad Arkhipov)
Список: pgsql-performance

Vlad Arkhipov <> 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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