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Re: Simple postgresql.conf wizard

From: "Robert Haas" <robertmhaas(at)gmail(dot)com>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Josh Berkus" <josh(at)agliodbs(dot)com>, "Gregory Stark" <stark(at)enterprisedb(dot)com>, "Greg Smith" <gsmith(at)gregsmith(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Simple postgresql.conf wizard
Date: 2008-11-27 22:15:04
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
> Even though we all agree default_statistics_target = 10 is too low,
> proposing a 40X increase in the default value requires more evidence
> than this.  In particular, the prospect of a 1600-fold increase in
> the typical cost of eqjoinsel() is a mite scary.

I just did some very quick testing of a couple values for
default_statistics_target on a database for work whose contents I am
not at liberty to share.  Suffice it to say it is a database with
small tables, but a lot of the queries involve many joins.

ANALYZE with default_statistics_target set to 10 takes 13 s.  With
100, 92 s.  With 1000, 289 s.

I tried several representative queries and there was no measurable
change in run-time.  I compared a couple of the plans and the plans
didn't change either.

So then I said - gee, how much can I reduce this?  I looked at two of
the queries in more detail.  The lowest default_statistics_target that
produced the same plan for both queries was 5.  Reducing the
default_statistics_target all the way down to 1 changed the plans, but
the new plans were just as fast as the old plans.

Given the amount of clamor for a higher value for
default_statistics_target, I'm a little surprised by these results.
It may be that the queries I'm running are not ones for which more
statistics generate better plans, but I think they really are
representative of what we run.  Maybe someone could suggest some types
of query that would be likely to helped by better statistics?

A random thought: maybe the reason I'm not seeing any benefit is
because my tables are just too small - most contain at most a few
thousand rows, and some are much smaller.  Maybe
default_statistics_target should vary with the table size?  Something
like, 0.1% of the rows to a maximum of 100...  and then 0.01% of the
rows after that to some higher maximum.... but always a minimum of at
least 10.  I'm guessing that people with really big tables are less
likely to mind longer planning times and more likely to benefit from
finding better plans...


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Subject: Re: Simple postgresql.conf wizard
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