To: "Brad Might" <bmight@storediq.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Specific query performance problem help requested - postgresql
	7.4
In-reply-to: <E387E2E9622FDD408359F98BF183879E08DA5C@dc1.storediq.com> 
References: <E387E2E9622FDD408359F98BF183879E08DA5C@dc1.storediq.com>
Comments: In-reply-to "Brad Might" <bmight@storediq.com>
	message dated "Thu, 26 May 2005 12:55:32 -0500"
Date: Thu, 26 May 2005 14:31:57 -0400
Message-ID: <4062.1117132317@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Brad Might" <bmight@storediq.com> writes:
> Can someone help me break this down and figure out why the one query
> takes so much longer than the other?

It looks to me like there's a correlation between filename and bucket,
such that the indexscan in filename order takes much longer to run
across the first 25 rows with bucket = 3 than it does to run across
the first 25 with bucket = 7 or bucket = 8.  It's not just a matter of
there being fewer rows with bucket = 3 ... the cost differential is much
larger than is explained by the count ratios.  The bucket = 3 rows have
to be lurking further to the back of the filename order than the others.

> Here's the bucket distribution..i have clustered the index on the bucket
> value.

If you have an index on bucket, it's not doing you any good here anyway,
since you wrote the constraint as a crosstype operator ("3" is int4 not
int8).  It might help to explicitly cast the constant to int8.

			regards, tom lane