From: Simon Riggs <simon@2ndquadrant.com>
To: Pablo Alcaraz <pabloa@laotraesquina.com.ar>
Cc: pgsql-performance@postgresql.org
Subject: Re: Speed difference between select ... union select ... and select from partitioned_table
Date: Sat, 27 Oct 2007 22:26:49 +0100
Message-ID: <1193520409.4242.604.camel@ebony.site> (raw)
In-Reply-To: <47225014.5040101@laotraesquina.com.ar>
References: <47225014.5040101@laotraesquina.com.ar>
On Fri, 2007-10-26 at 16:37 -0400, Pablo Alcaraz wrote:
> I executed 2 equivalents queries. The first one uses a union structure.
> The second uses a partitioned table. The tables are the same with 30
> millions of rows each one and the returned rows are the same.
>
> But the union query perform faster than the partitioned query.
>
> My question is: why? :)
The two queries are equivalent but they have different execution plans.
The UNION query has explicit GROUP BY operations within it. We do not
currently perform a push-down operation onto the individual partitions.
This results in more data copying as well as requiring a single very
large sort, rather than lots of small ones. That is probably enough to
allow it to perform the sort in memory rather than on-disk, thus
allowing a considerable speed-up.
This is on my list of requirements for further partitioning improvements
in 8.4 or beyond.
--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com
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To: pgsql-performance@postgresql.org
Cc: simon@2ndquadrant.com, pabloa@laotraesquina.com.ar
Subject: Re: Speed difference between select ... union select ... and select from partitioned_table
In-Reply-To: <1193520409.4242.604.camel@ebony.site>
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