To: Jerry Gamache <jerry.gamache@idilia.com>
cc: pgsql-performance@postgresql.org
Subject: Re: Logarithmic data frequency distributions and the query planner 
In-reply-to: <4C34E998.3020000@idilia.com> 
References: <4C34E998.3020000@idilia.com>
Comments: In-reply-to Jerry Gamache <jerry.gamache@idilia.com>
	message dated "Wed, 07 Jul 2010 16:54:48 -0400"
Date: Wed, 07 Jul 2010 17:22:00 -0400
Message-ID: <16024.1278537720@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Jerry Gamache <jerry.gamache@idilia.com> writes:
> On 8.1, I have a very interesting database where the distributions of 
> some values in a multi-million rows table is logarithmic (i.e. the most 
> frequent value is an order of magnitude more frequent than the next 
> ones). If I analyze the table, the statistics become extremely skewed 
> towards the most frequent values and this prevents the planner from 
> giving any good results on queries that do not target these entries.

Highly skewed distributions are hardly unusual, and I'm not aware that
the planner is totally incapable of dealing with them.  You do need a
large enough stats target to get down into the tail of the
distribution (the default target for 8.1 is probably too small for you).
It might be that there have been some other relevant improvements since
8.1, too ...

			regards, tom lane