Andrew - Supernews <andrew+nonews(at)supernews(dot)com> writes:
> On 2006-12-13, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> BTW, I think it's possible to prove that there need never be two for the
>> case of both sides the same datatype.
> Counterexample even for a single data type: define an operator x =* y
> which is true when 2x = y. This is mergejoinable using the following
> operators: SORT1 = <, SORT2 = <, LTCMP = (2x < y), RTCMP = (2x > y)
> (which is of course the same sortops as for regular =).
I think not --- the corresponding sort operators would have to be
"2x < y" etc, else the trichotomy law fails, and so do all standard
sort algorithms.
regards, tom lane