Re: XMIN/xid vs UNION

Karsten Hilbert <Karsten(dot)Hilbert(at)gmx(dot)net> writes:
> However, the question still holds true: Is there any
> particular reason those operators aren't found in my PG
> installation despite being listed as added since 7.2 ?

The only thing that was added in 7.2 was xid equality.

There was some talk recently of adding the other comparison operations,
but I'm not sure it's a good idea. For many purposes you'd want such
comparison operators to use the same semantics as TransactionIdPrecedes
(ie, compare mod 2^31) but that would mean that some of the usual
arithmetic laws break down (no transitive law, no triangle inequality)
and in particular you could not sort or build a btree index with such
operators. Which in turn means that you still wouldn't get the behavior
you wanted of having UNION or ORDER BY "just work".

regards, tom lane