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From: Hannu Krosing <hannu@2ndquadrant.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Peter Hussey <peter@labkey.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: Questions on query planner, join types, and work_mem
Date: Wed, 04 Aug 2010 20:51:08 +0300
Message-ID: <1280944268.3735.12.camel@hvost> (raw)
In-Reply-To: <AANLkTik6UBt2ezVFzdCM8iaUPkra_7f-Hs_AmGZgvFhh@mail.gmail.com>
References: <AANLkTikMwVZKDYLitnsK31R=VNOuTH8NtmDHDGkBR7k8@mail.gmail.com>
	<18551.1280275502@sss.pgh.pa.us>
	<AANLkTimXMR0jMdw+66E4Xx=SqNV37Ne9WfkFjPFZVjbV@mail.gmail.com>
	<25534.1280498632@sss.pgh.pa.us>
	<AANLkTik3oY1=F_uC843c84LxBLNdHxZaWdfjxrAtyBeu@mail.gmail.com>
	<1280819031.14579.38.camel@hvost>
	<AANLkTik6UBt2ezVFzdCM8iaUPkra_7f-Hs_AmGZgvFhh@mail.gmail.com>

On Wed, 2010-08-04 at 09:14 -0400, Robert Haas wrote:
> On Tue, Aug 3, 2010 at 3:03 AM, Hannu Krosing <hannu@2ndquadrant.com> wrote:
> > In case of fully cached database it is closer to 1.
> 
> In the case of a fully cached database I believe the correct answer
> begins with a decimal point.

The number 1 here was suggested in relation to seq_page_cost, which is
1. 

For fully cached db there is no additional seek time for random access,
so seq_page_cost == random_page_cost.

Of course there are more variables than just *_page_cost, so if you nail
down any other one, you may end with less than 1 for both page costs.

I have always used seq_page_cost = 1 in my thinking and adjusted others
relative to it.

-- 
Hannu Krosing   http://www.2ndQuadrant.com
PostgreSQL Scalability and Availability 
   Services, Consulting and Training





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