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Re: VACUUM ANALYZE is faster than ANALYZE?

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Robert Haas <robertmhaas(at)gmail(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: 2012-02-23 08:34:18
Message-ID: CA+U5nMKUGRz9EJQoig-pJqWcdPE3haZC8iV+3nXK8MvHz5q8mg@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, Feb 22, 2012 at 10:02 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> 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.

PostgreSQL already uses  a parameter called "random_page_cost" to
describe non-sequential access. Perhaps that is wrong and we need a
third parameter?

> 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.

By that argument, ANALYZE never could run longer than VACUUM ANALYZE,
so you disagree with Tom and I and you can't explain Pavel's
results....

cost_bitmap_heap_scan() uses "random_page_cost" to evaluate the cost
of accessing blocks, even though the author knew the access was in
ascending block number order. Why was that?

Note that the cost_bitmap_heap_scan() cost can be > than
cost-seqscan() for certain parameter values.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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