To: Hannu Krosing <hannu@2ndquadrant.com>
cc: Robert Haas <robertmhaas@gmail.com>, Peter Hussey <peter@labkey.com>,
	pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: Questions on query planner, join types, and work_mem 
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Comments: In-reply-to Hannu Krosing <hannu@2ndquadrant.com>
	message dated "Wed, 04 Aug 2010 20:51:08 +0300"
Date: Wed, 04 Aug 2010 14:00:36 -0400
Message-ID: <22961.1280944836@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Hannu Krosing <hannu@2ndquadrant.com> writes:
> 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.

Right, seq_page_cost = 1 is sort of the traditional reference point,
but you don't have to do it that way.  The main point here is that for
an all-in-RAM database, the standard page access costs are too high
relative to the CPU effort costs:

regression=# select name, setting from pg_settings where name like '%cost';
         name         | setting 
----------------------+---------
 cpu_index_tuple_cost | 0.005
 cpu_operator_cost    | 0.0025
 cpu_tuple_cost       | 0.01
 random_page_cost     | 4
 seq_page_cost        | 1
(5 rows)

To model an all-in-RAM database, you can either dial down both
random_page_cost and seq_page_cost to 0.1 or so, or set random_page_cost
to 1 and increase all the CPU costs.  The former is less effort ;-)

It should be noted also that there's not all that much evidence backing
up the default values of the cpu_xxx_cost variables.  In the past those
didn't matter much because I/O costs always swamped CPU costs anyway.
But I can foresee us having to twiddle those defaults and maybe refine
the CPU cost model more, as all-in-RAM cases get more common.

			regards, tom lane