[lmm]

> You can have a table or view of all the ids across your implementations and FK into that.

That's just moving the problem around. That table would have to have a bunch of nullable columns, and there's no way to express the constraint that either these columns are non-null or those columns are non-null, or these columns are non-null when this enum has this value.

I'm guessing you've never seriously used a language with first-class sum types. Yes, you can use hacks to represent sum-typed data in languages that don't have proper sum types, but it's always going to be a hack. It's like saying C has OO support because you can always construct virtual function tables by hand.