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From: Kevin Grittner <kgrittn@ymail.com>
To: ach <alanchines@gmail.com>
To: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: statistics target for columns in unique constraint?
Date: Mon, 20 May 2013 07:12:20 -0700 (PDT)
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ach <alanchines@gmail.com> wrote:
> One quick follow up since I'm expecting y'all might know: Do the
> statistics targets actually speed performance on an index search
> itself; the actual lookup? Or are the JUST to inform the planner
> towards the best pathway decision?
Since the statistics are just a random sampling and generally not
completely up-to-date, they really can't be used for anything other
than *estimating* relative costs in order to try to pick the best
plan. Once a plan is chosen, its execution time is not influenced
by the statistics. A higher statistics target can increase
planning time. In a complex query with many joins and many indexes
on the referenced tables, the increase in planning time can be
significant. I have seen cases where blindly increasing the
default statistics target resulted in planning time which was
longer than run time -- without any increase in plan quality.
Generally when something is configurable, it's because there can be
benefit to adjusting it. If there was a single setting which could
not be materially improved upon for some cases, we wouldn't expose
a configuration option. This is something which is not only
globally adjustable, you can override the setting for individual
columns -- again, we don't go to the trouble of supporting that
without a good reason.
--
Kevin Grittner
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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