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Date: Wed, 3 Mar 2010 12:12:40 -0500
Message-ID: <603c8f071003030912k5750abadif64cb36d67ab99ae@mail.gmail.com>
Subject: Re: Estimation issue with partitioned tables
From: Robert Haas <robertmhaas@gmail.com>
To: Josh Berkus <josh@agliodbs.com>
Cc: pgsql-performance@postgresql.org
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On Sun, Feb 28, 2010 at 3:19 PM, Josh Berkus <josh@agliodbs.com> wrote:
> All,
>
> I'm seeing in a production database two problems with query rowcount
> estimation:
>
> (1) Estimates for the number of rows in an outer join do not take into
> account any constraint exclusion (CE) in operation.
>
> (2) Row estimates do not take into account if the unique indexes on the
> child partitions are different from the master partition (the append
> node). =A0This is often true, because the key to the master is ( key, ce
> column) and for the children is just ( key ).
>
> The result is that if you do a series of outer joins using the CE
> criterion against partitioned tables, the row estimates you get will be
> several orders of magnitude too high ... and the subsequent query plan
> far too pessimistic.
>
> Anyone else seeing this? =A0Do any of the 9.0 patches address the above
> issues?

I feel like I've seen these way-too-high row estimates in some other
postings to -performance, but I'm not sure if it was the same issue.
You don't by chance have a RTC? I don't think it's likely fixed in 9.0
but it would be interesting to investigate.

...Robert