From: Tom Lane <tgl@sss.pgh.pa.us>
To: Jeremy Palmer <JPalmer@linz.govt.nz>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Possible to improve query plan?
Date: Mon, 17 Jan 2011 15:24:26 -0500
Message-ID: <20065.1295295866@sss.pgh.pa.us> (raw)
In-Reply-To: <666FB8D75E95AE42965A0E76A5E5337E06D2C683BF@prdlsmmsg01.ad.linz.govt.nz>
References: <666FB8D75E95AE42965A0E76A5E5337E06D2C683BF@prdlsmmsg01.ad.linz.govt.nz>
Jeremy Palmer <JPalmer@linz.govt.nz> writes:
> I've come to a dead end in trying to get a commonly used query to
> perform better.
> EXPLAIN
> SELECT * FROM (
> SELECT
> row_number() OVER (PARTITION BY id ORDER BY _revision_created DESC) as row_number,
> *
> FROM
> version_crs_coordinate_revision
> WHERE (
> (_revision_created <= 16 AND _revision_expired > 16 AND _revision_expired <= 40) OR
> (_revision_created > 16 AND _revision_created <= 40)
> )
> ) AS T
> WHERE row_number = 1;
If I'm not mistaken, that's a DB2-ish locution for a query with DISTINCT
ON, ie, you're looking for the row with highest _revision_created for
each value of id. It might perform well on DB2, but it's going to
mostly suck on Postgres --- we don't optimize window-function queries
very much at all at the moment. Try writing it with DISTINCT ON instead
of a window function, like so:
SELECT DISTINCT ON (id)
*
FROM
version_crs_coordinate_revision
WHERE (
(_revision_created <= 16 AND _revision_expired > 16 AND _revision_expired <= 40) OR
(_revision_created > 16 AND _revision_created <= 40)
)
ORDER BY id, _revision_created DESC;
You could also experiment with various forms of GROUP BY if you're loath
to use any Postgres-specific syntax.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, JPalmer@linz.govt.nz
Subject: Re: Possible to improve query plan?
In-Reply-To: <20065.1295295866@sss.pgh.pa.us>
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