Re: Allow foreign keys to reference a superset of unique columns

James Coleman:
> As I was reading through the email chain I had this thought: could you
> get the same benefit (or 90% of it anyway) by instead allowing the
> creation of a uniqueness constraint that contains more columns than
> the index backing it? So long as the index backing it still guaranteed
> the uniqueness on a subset of columns that would seem to be safe.
>
> Tom notes the additional columns being nullable is something to think
> about. But if we go the route of allowing unique constraints with
> backing indexes having a subset of columns from the constraint I don't
> think the nullability is an issue since it's already the case that a
> unique constraint can be declared on columns that are nullable. Indeed
> it's also the case that we already support a foreign key constraint
> backed by a unique constraint including nullable columns.
>
> Because such an approach would, I believe, avoid changing the foreign
> key code (or semantics) at all, I believe that would address Tom's
> concerns about information_schema and fuzziness of semantics.

Could we create this additional unique constraint implicitly, when using
FOREIGN KEY ... REFERENCES on a superset of unique columns? This would
make it easier to use, but still give proper information_schema output.

Best

Wolfgang