Re: Specific query performance problem help requested - postgresql 7.4

От: Tom Lane
Тема: Re: Specific query performance problem help requested - postgresql 7.4
Дата: ,
Msg-id: 4062.1117132317@sss.pgh.pa.us
(см: обсуждение, исходный текст)
Ответ на: Specific query performance problem help requested - postgresql 7.4  ("Brad Might")
Список: pgsql-performance

"Brad Might" <> 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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