| 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 |
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| 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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