On 2006-12-13, Tom Lane <[EMAIL PROTECTED]> wrote: > Andrew - Supernews <[EMAIL PROTECTED]> writes: >> On 2006-12-13, Tom Lane <[EMAIL PROTECTED]> 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.
No, because if x < x' then 2x < 2x'. Or to put it another way, doing a merge join on (2x = y) simply requires matching the sorted lists of x's and y's against each other in a different place, rather than changing the sort order of either. You're suffering from a fundamental confusion between the ltcmp/rtcmp operators (which indeed must be trichotomous with the join operator) and the sort operators. -- Andrew, Supernews http://www.supernews.com - individual and corporate NNTP services ---------------------------(end of broadcast)--------------------------- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
