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Re: variance aggregates per SQL:2003

From: Neil Conway <neilc(at)samurai(dot)com>
To: David Fetter <david(at)fetter(dot)org>
Cc: pgsql-patches <pgsql-patches(at)postgresql(dot)org>
Subject: Re: variance aggregates per SQL:2003
Date: 2006-03-08 00:56:06
Message-ID: 1141779366.20504.17.camel@localhost.localdomain (view raw or flat)
Thread:
Lists: pgsql-patches
On Tue, 2006-03-07 at 16:36 -0800, David Fetter wrote:
> The rationale is kinda mathematical.  A measure of deviation from
> central tendency (i.e. variance or stddev) is something where you
> probably don't want to normalize the weights.
> 
> For example, the standard deviation of {0,1,1,1,2} is about 0.707, but
> the standard deviation of {0,1,2} is 1.

Well, I realize that stddev(DISTINCT x) != stddev(x) and that most
people are going to be interested in stddev(x), but I don't think it's
inconceivable for someone to be interested in stddev(DISTINCT x).
Explicitly checking for and rejecting it doesn't serve any useful
purpose that I can see, beyond compliance with the letter of the
standard -- if the user asks for stddev(DISTINCT x), are we really
providing useful behavior if we refuse to calculate it?

-Neil



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