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Re: large tables and simple "= constant" queries using indexes

From: Bill Moran <wmoran(at)collaborativefusion(dot)com>
To: Arjen van der Meijden <acmmailing(at)tweakers(dot)net>
Cc: John Beaver <john(dot)e(dot)beaver(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: large tables and simple "= constant" queries using indexes
Date: 2008-04-09 21:36:09
Message-ID: 20080409173609.d1b959c0.wmoran@collaborativefusion.com (view raw or flat)
Thread:
Lists: pgsql-performance
This is a FAQ, it comes up on an almost weekly basis.  Please do a
little Googling on count(*) and PostgreSQL and you'll get all the
explanations and suggestions on how to fix the problem you could
ever want.

In response to Arjen van der Meijden <acmmailing(at)tweakers(dot)net>:

> First of all, there is the 'explain analyze' output, which is pretty 
> helpful in postgresql.
> 
> My guess is, postgresql decides to do a table scan for some reason. It 
> might not have enough statistics for this particular table or column, to 
> make a sound decision. What you can try is to increase the statistics 
> target, which works pretty easy:
> ALTER TABLE gene_prediction_view ALTER gene_ref SET STATISTICS 200;
> 
> Valid ranges are from 1(0?) - 1000, the default is 10, the default on my 
> systems is usually 100. For such a large table, I'd go with 200.
> 
> After that, you'll need to re-analyze your table and you can try again.
> 
> Perhaps analyze should try to establish its own best guess to how many 
> samples it should take? The default of 10 is rather limited for large 
> tables.
> 
> Best regards,
> 
> Arjen
> 
> On 9-4-2008 22:58 John Beaver wrote:
> > Hi, I've started my first project with Postgres (after several years of 
> > using Mysql), and I'm having an odd performance problem that I was 
> > hoping someone might be able to explain the cause of.
> > 
> > ----My query----
> >    - select count(*) from gene_prediction_view where gene_ref = 523
> >    - takes 26 seconds to execute, and returns 2400 (out of a total of 15 
> > million records in the table)
> > 
> > ---My problem---
> >    Using a single-column index to count 2400 records which are exactly 
> > one constant value doesn't sound like something that would take 26 
> > seconds. What's the slowdown? Any silver bullets that might fix this?
> > 
> > ----Steps I've taken----
> >    - I ran vacuum and analyze
> >    - I upped the shared_buffers to 58384, and I upped some of the other 
> > postgresql.conf values as well. Nothing seemed to help significantly, 
> > but maybe I missed something that would help specifically for this query 
> > type?
> >    - I tried to create a hash index, but gave up after more than 4 hours 
> > of waiting for it to finish indexing
> > 
> > ----Table stats----
> >    - 15 million rows; I'm expecting to have four or five times this 
> > number eventually.
> >    - 1.5 gigs of hard drive usage
> > 
> > ----My development environment---
> >    - 2.6ghz dual-core MacBook Pro with 4 gigs of ram and a 7200 rpm hard 
> > drive
> >    - OS X 10.5.2
> >    - Postgres 8.3 (installed via MacPorts)
> > 
> > ----My table----
> > 
> > CREATE TABLE gene_prediction_view
> > (
> >  id serial NOT NULL,
> >  gene_ref integer NOT NULL,
> >  go_id integer NOT NULL,
> >  go_description character varying(200) NOT NULL,
> >  go_category character varying(50) NOT NULL,
> >  function_verified_exactly boolean NOT NULL,
> >  function_verified_with_parent_go boolean NOT NULL,
> >  function_verified_with_child_go boolean NOT NULL,
> >  score numeric(10,2) NOT NULL,
> >  precision_score numeric(10,2) NOT NULL,
> >  CONSTRAINT gene_prediction_view_pkey PRIMARY KEY (id),
> >  CONSTRAINT gene_prediction_view_gene_ref_fkey FOREIGN KEY (gene_ref)
> >      REFERENCES sgd_annotations (id) MATCH SIMPLE
> >      ON UPDATE NO ACTION ON DELETE NO ACTION,
> >  CONSTRAINT gene_prediction_view_go_id_fkey FOREIGN KEY (go_id)
> >      REFERENCES go_terms (term) MATCH SIMPLE
> >      ON UPDATE NO ACTION ON DELETE NO ACTION,
> >  CONSTRAINT gene_prediction_view_gene_ref_key UNIQUE (gene_ref, go_id)
> > )
> > WITH (OIDS=FALSE);
> > ALTER TABLE gene_prediction_view OWNER TO postgres;
> > 
> > CREATE INDEX ix_gene_prediction_view_gene_ref
> >  ON gene_prediction_view
> >  USING btree
> >  (gene_ref);
> > 
> > 
> > 
> > 
> 
> -- 
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> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-performance


-- 
Bill Moran
Collaborative Fusion Inc.
http://people.collaborativefusion.com/~wmoran/

wmoran(at)collaborativefusion(dot)com
Phone: 412-422-3463x4023

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Next:From: Jeremy HarrisDate: 2008-04-09 22:07:50
Subject: Re: large tables and simple "= constant" queries using indexes
Previous:From: Arjen van der MeijdenDate: 2008-04-09 21:21:20
Subject: Re: large tables and simple "= constant" queries using indexes

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