What's the reason you think you need another ID column, Tim? .... There may be other reasons for you trying to partition your PK, if you told us why, then maybe someone may come up with a simpler way to solve your problem than add another part to the PK.

It's a long complicated story - this particular approach is the one that involves redesigning as little as possible of the overall system architecture. Other approaches are, indeed, no doubt, possible, but I'm starting with the one(s) that appear to give (even with the problems I've identified so far) the shortest implementation time.

(It probably doesn't help that I'm more used to other databases which seem less keen on stopping you doing things because of dependencies.)

-- 
Tim Ward