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 |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
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
From | Date | Subject | |
---|---|---|---|
Next Message | David Fetter | 2006-03-08 00:58:47 | Re: variance aggregates per SQL:2003 |
Previous Message | David Fetter | 2006-03-08 00:36:04 | Re: variance aggregates per SQL:2003 |