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Re: Specific query performance problem help requested - postgresql 7.4

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Brad Might" <bmight(at)storediq(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Specific query performance problem help requested - postgresql 7.4
Date: 2005-05-26 18:31:57
Message-ID: 4062.1117132317@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
"Brad Might" <bmight(at)storediq(dot)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

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Next:From: Brad MightDate: 2005-05-26 18:41:44
Subject: Re: Specific query performance problem help requested - postgresql 7.4
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Subject: Re: Optimising queries involving unions

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