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Re: planner costs in "warm cache" tests

From: Jesper Krogh <jesper(at)krogh(dot)cc>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: planner costs in "warm cache" tests
Date: 2010-05-31 18:48:40
Message-ID: 4C040488.30906@krogh.cc (view raw or flat)
Thread:
Lists: pgsql-performance
On 2010-05-30 20:34, Tom Lane wrote:
> Jesper Krogh<jesper(at)krogh(dot)cc>  writes:
>    
>> testdb=# set seq_page_cost = 0.00001;
>> SET
>> testdb=# set random_page_cost = 0.00001;
>> SET
>>      
> Well, hmm, I really doubt that that represents reality either.  A page
> access is by no means "free" even when the page is already in cache.
> I don't recall anyone suggesting that you set these numbers to less
> than perhaps 0.01.
>
>    
Thank you for the prompt response. Is it a "false assumption" that the
cost should in some metric between different plans be a measurement
of actual run-time in a dead-disk run?

It should most likely be matching a typical workload situation, but that
it really hard to tell anything about, so my "feeling" would be that the
dead disk case is the one closest?

-- 
Jesper

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