On Mon, May 22, 2006 at 10:23:28 +0200,
Albe Laurenz <all(at)adv(dot)magwien(dot)gv(dot)at> wrote:
>
> In order to (efficiently) process a GROUP BY clause, you need a
> total ordering on the data type that you group by, i.e. an ordering
> such that for any two data x and y you have either x < y or x > x
> or x = y.
An equality operator is good enough if the number of unique groups isn't too
large, so that a hash aggregate plan works efficiently.