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Re: How to optimize monstrous query, sorts instead of

From: Rod Taylor <rbt(at)rbt(dot)ca>
To: michael(dot)mattox(at)verideon(dot)com
Cc: Postgresql Performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: How to optimize monstrous query, sorts instead of
Date: 2003-06-25 12:12:09
Message-ID: 1056543129.69651.82.camel@jester (view raw or flat)
Thread:
Lists: pgsql-performance
> Here's the output of explain analyze.  The query typically takes 0-4 seconds
> depending on the time frame.  It's run very frequently especially to process
> the nightly reports.

The plan picked seems reasonable (estimated costs / tuples is close to
actual).

I think the biggest hit is this index scan.  Thats a substantial cost to
pull out less than a thousand lines:

                           ->  Index Scan using monitorstatusxmonitori
on
monitorstatusx ms  (cost=0.00..4695.65 rows=880 width=83) (actual
time=40.17..1868.12 rows=625 loops=1)
                                 Index Cond: ("outer".jdoidx =
ms.monitorx)
                                 Filter: ((datex >= '2003-06-20
08:57:21.36'::timestamp without time zone) AND (datex <= '2003-06-29
08:57:21.36'::timestamp without time zone))


Are jdoidx and monitorx integers?

You might try a multi-column index on (ms.monitorx, ms.datex).

Are monitorx assigned roughly ordered by date?  It must be, otherwise
the sort step would not be so cheap (hardly any impact on the query --
see actual cost number).  The multi-column index above should give you a
bit of a boost.

Depending on the data in the table, the index (ms.datex, monitorx) may
give better results along with a single index on (ms.monitorx) as you
currently have.  It's not very likely though.

-- 
Rod Taylor <rbt(at)rbt(dot)ca>

PGP Key: http://www.rbt.ca/rbtpub.asc

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Subject: Re: How to optimize monstrous query, sorts instead of
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