From: Kevin Grittner <kgrittn@ymail.com>
To: Torsten Förtsch <torsten.foertsch@gmx.net>
To: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: planner parameters
Date: Tue, 3 Sep 2013 12:02:00 -0700 (PDT)
Message-ID: <1378234920.66126.YahooMailNeo@web162906.mail.bf1.yahoo.com> (raw)
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Torsten Förtsch <torsten.foertsch@gmx.net> wrote:
> Is there an other way to make the planner use generate the 1st
> plan?
The planner cost factors are based on the assumption that a
moderate percentage of random page reads will need to actually go
out to disk. If a high percentage of pages are in cache, you may
want to reduce random_page_cost to something closer to (or even
equal to) seq_page_cost. I generally find I get better plans if I
raise cpu_tuple_cost to 0.03. effective_cache_size should
generally be between 50% and 75% of machine RAM. If these changes
(or others of their ilk) cause costs to be estimated in a way which
more nearly matches reality, better plans will be chosen.
> Why does it generate the 2nd plan at all?
It has the lowest estimated cost, based on your memory
configuration and cost factors.
> Does the planner take into account what is currently present in
> shared memory?
No. If you search the archives you can probably find previous
discussions of whether it would be a good idea to do so; the
consensus has been that it would not be.
If you have further performance-related questions, please review
this page so that you can provide enough information to allow
people to give the most helpful advice:
http://wiki.postgresql.org/wiki/SlowQueryQuestions
--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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To: pgsql-performance@postgresql.org
Cc: kgrittn@ymail.com, torsten.foertsch@gmx.net
Subject: Re: planner parameters
In-Reply-To: <1378234920.66126.YahooMailNeo@web162906.mail.bf1.yahoo.com>
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