From: Moritz Lennert <mlennert@club.worldonline.be>
To: pgsql-sql@postgresql.org
Subject: performance question
Date: Mon, 20 Jan 2003 12:40:34 +0100 (CET)
Message-ID: <59665.164.15.128.4.1043062834.squirrel@http://moritz.homelinux.org> (raw)
Hello,
I have a table with some 2.2 million rows on a Pentium4, 1.8GHz with 512
MB RAM.
Some queries I launch take quite a long time, and I'm wondering whether
this is normal,or whether I can get better performance somehow.
As an example, I have a field which is in char(2), with 23 different
possible values. When I launch a "select * from table where field = 'xx'"
this takes a very long time (some 230194.10 msec, i.e. almost 4 minutes,
according to "explain analyze"). I've tried indexing on this column and
vacuuming ("vacuum analyze"), but this doesn't change anything. "explain
select" always gives me a sequential scan. The correlation value in
pg_stats for this column is 0.0583268, so a seq scan is probably cheaper
than index scan.
Now maybe this is just the way it is and there is no possibility to
enhance performance, but if someone has some tips on what I might try,
they are very welcome !
One question I asked myself is whether the use of char(2) is the best
option. The column (and most others in the table) contains codes that
designate different characteristics (for ex. in a column 'sex' one would
find '1'=male, '2'=female).
Moritz
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To: pgsql-sql@postgresql.org
Cc: mlennert@club.worldonline.be
Subject: Re: performance question
In-Reply-To: <59665.164.15.128.4.1043062834.squirrel@http://moritz.homelinux.org>
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