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From: Jesper Krogh <jesper@krogh.cc>
To: Greg Smith <greg@2ndquadrant.com>
Cc: Craig Ringer <craig@postnewspapers.com.au>
Cc: Cédric Villemain <cedric.villemain.debian@gmail.com>
Cc: Josh Berkus <josh@agliodbs.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: reducing random_page_cost from 4 to 2 to force index scan
Date: Mon, 16 May 2011 06:41:58 +0200
Message-ID: <4DD0AB16.5060808@krogh.cc> (raw)
In-Reply-To: <4DD07B4E.1020503@2ndquadrant.com>
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On 2011-05-16 03:18, Greg Smith wrote:
>  You can't do it in real-time. You don't necessarily want that to
>  even if it were possible; too many possibilities for nasty feedback
>  loops where you always favor using some marginal index that happens
>  to be in memory, and therefore never page in things that would be
>  faster once they're read. The only reasonable implementation that
>  avoids completely unstable plans is to scan this data periodically
>  and save some statistics on it--the way ANALYZE does--and then have
>  that turn into a planner input.

Would that be feasible? Have process collecting the data every now-and-then
probably picking some conservative-average function and feeding
it into pg_stats for each index/relation?

To me it seems like a robust and fairly trivial way to to get better 
numbers. The
fear is that the OS-cache is too much in flux to get any stable numbers out
of it.

-- 
Jesper

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