| From: | Hannu Krosing <hannu(at)2ndquadrant(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Peter Hussey <peter(at)labkey(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-performance <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: Questions on query planner, join types, and work_mem |
| Date: | 2010-08-04 17:51:08 |
| Message-ID: | 1280944268.3735.12.camel@hvost |
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| Thread: | |
| Lists: | pgsql-performance |
On Wed, 2010-08-04 at 09:14 -0400, Robert Haas wrote:
> On Tue, Aug 3, 2010 at 3:03 AM, Hannu Krosing <hannu(at)2ndquadrant(dot)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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