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Re: VACUUM ANALYZE is faster than ANALYZE?

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: VACUUM ANALYZE is faster than ANALYZE?
Date: 2012-02-22 22:02:43
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Wed, Feb 22, 2012 at 2:23 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> The industry accepted description for non-sequential access is "random
> access" whether or not the function that describes the movement is
> entirely random. To argue otherwise is merely hairsplitting.

I don't think so.  For example, a bitmap index scan contrives to speed
things up by arranging for the table I/O to happen in ascending block
number order, with skips, rather than in random order, as a plain
index scan would do, and that seems to be a pretty effective
technique.  Except to the extent that it interferes with the kernel's
ability to do readahead, it really can't be to read blocks 1, 2, 3, 4,
and 5 than to read blocks 1, 2, 4, and 5.  Not reading block 3 can't
require more effort than reading it.

Robert Haas
The Enterprise PostgreSQL Company

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