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From: Jesper Krogh <jesper@krogh.cc>
To: pgsql-performance@postgresql.org
Subject: Re: reducing random_page_cost from 4 to 2 to force index scan
Date: Mon, 16 May 2011 06:49:20 +0200
Message-ID: <4DD0ACD0.1090801@krogh.cc> (raw)
In-Reply-To: <4DD0AB16.5060808@krogh.cc>
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On 2011-05-16 06:41, Jesper Krogh wrote:
> 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.

Ok, it may not work as well with index'es, since having 1% in cache may very
well mean that 90% of all requested blocks are there.. for tables in should
be more trivial.

-- 
Jesper



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