From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Hannu Krosing <hannu(at)2ndquadrant(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Peter Hussey <peter(at)labkey(dot)com>, pgsql-performance <pgsql-performance(at)postgresql(dot)org> |
Subject: | Re: Questions on query planner, join types, and work_mem |
Date: | 2010-08-04 18:00:36 |
Message-ID: | 22961.1280944836@sss.pgh.pa.us |
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Lists: | pgsql-performance |
Hannu Krosing <hannu(at)2ndquadrant(dot)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
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