pg.ddx.io  pgsql-performance@postgresql.org mailing list archive  

help / color / mirror / Atom feed
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Greg Smith <greg@2ndquadrant.com>
Cc: jd@commandprompt.com, Ogden <lists@darkstatic.com>
Cc: Jesper Krogh <jesper@krogh.cc>
Cc: Kenneth Marshall <ktm@rice.edu>
Cc: pgsql-performance@postgresql.org
Subject: Re: Query much faster with enable_seqscan=0
Date: Tue, 21 Sep 2010 19:24:47 -0400
Message-ID: <11632.1285111487@sss.pgh.pa.us> (raw)
In-Reply-To: <4C9904A9.5080301@2ndquadrant.com>
References: <0D5B9ED0-00C3-4EEB-8336-0B2F8A884620@darkstatic.com>
	<20100921180618.GI17152@aart.is.rice.edu>
	<D707045F-C689-403A-9729-8DBCC39F53A1@darkstatic.com>
	<4C98FEA2.8030607@krogh.cc>
	<3062DF4E-7460-4487-818A-BF77F176E501@darkstatic.com>
	<1285096031.15919.68.camel@jd-desktop.unknown.charter.com>
	<4C9904A9.5080301@2ndquadrant.com>

Greg Smith <greg@2ndquadrant.com> writes:
> and the query optimizer needs to be careful about what it does and 
> doesn't pull from disk.  If that's not the case, like here where there's 
> 8GB of RAM and a 7GB database, dramatic reductions to both seq_page_cost 
> and random_page_cost can make sense.  Don't be afraid to think lowering 
> below 1.0 is going too far--something more like 0.01 for sequential and 
> 0.02 for random may actually reflect reality here.

If you are tuning for an all-in-RAM situation, you should set
random_page_cost equal to seq_page_cost (and usually set both smaller
than 1).  By definition, those costs are equal if you're fetching from
RAM.  If it's only mostly-in-RAM then keeping random_page_cost a bit
higher makes sense.

			regards, tom lane



view thread (16+ messages)  latest in thread

Message-ID: <11632.1285111487@sss.pgh.pa.us>
Permalink:  ../11632.1285111487@sss.pgh.pa.us/
Also on:    postgresql.org/message-id/11632.1285111487@sss.pgh.pa.us

 · 

reply
Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: pgsql-performance@postgresql.org
  Cc: tgl@sss.pgh.pa.us, greg@2ndquadrant.com, lists@darkstatic.com, jesper@krogh.cc, ktm@rice.edu
  Subject: Re: Query much faster with enable_seqscan=0
  In-Reply-To: <11632.1285111487@sss.pgh.pa.us>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox