> What datatype exactly? Integer, Numeric(18,0) or BigInt?
>
> Numeric(18,0) and BigInt is a 64-bit Integer. I doubt you reach the
> limit. With Integer (a 32-bit Integer) this is another story. I would
> rather go with a 64-bit Integer instead of a GUID in that scenario.

The PK is 18,0, which I know is 64-bit, but GUID is 128-bit. Unless it is contraindicated, because of performance or something else, I'd rather go bigger. When I said the app is active, I wasn't kidding. In addition to being active, I've had users of the previous version for over 15 years. I know than 900 trillion is big (64-bit), but if one of my customers gets there, it will be without warning. That means I'll have to drop everything an invent a replacement database, and data migration tool. Not something I wish to do while a customer is down, and unable to conduct normal business.

My question still stands.