Re: Specific query performance problem help requested - postgresql 7.4

How is it that the index scan has such poor performance? Shouldn't index
lookups be quicker?
-----Original Message-----
From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
Sent: Thursday, May 26, 2005 1:32 PM
To: Brad Might
Cc: pgsql-performance@postgresql.org
Subject: Re: [PERFORM] Specific query performance problem help requested
- postgresql 7.4

"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