From: Neil Conway <neilc@samurai.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgresql.org
Subject: Re: raising the default default_statistics_target
Date: Sun, 07 Mar 2004 18:20:51 -0500
Message-ID: <404BAE53.1000709@samurai.com> (raw)
In-Reply-To: <23289.1078700399@sss.pgh.pa.us>
References: <404B9747.4080702@samurai.com>
<23289.1078700399@sss.pgh.pa.us>
Tom Lane wrote:
> This is something we need to consider, but we'll need more evidence
> before making a choice. One thing that we have very little data about
> is how much difference it makes in the quality of planner choices.
Right, but is there a practical way to actually get this data?
If the distribution of data in the table is irregular, a higher stats
target is needed to allow good planning choices. Therefore, the effect
that the stats target has on planner choices depends on the regularity
of the distribution of data at installations, and there is no way to
know that in general AFAICS.
> Also, I would expect that larger stats targets would slow down the parts
> of the planner that look at the stats, since there are more data values
> to examine. I do not have any numbers about this cost though --- do you
> want to try to get some?
Given the magnitude of the change (25 data elements versus 10), I
wouldn't expect this to produce a major change in the total runtime of
the optimizer. However, I don't know the optimizer that well, so I'll
do some benchmarks when I get a chance.
-Neil
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To: pgsql-hackers@postgresql.org
Cc: neilc@samurai.com, tgl@sss.pgh.pa.us
Subject: Re: raising the default default_statistics_target
In-Reply-To: <404BAE53.1000709@samurai.com>
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