Re: foreign keys for array/period contains relationships

Example #4: PK is period, FK is timestamp.  FK must be contained in some
PK period.

CREATE TABLE pk (a period PRIMARY KEY, ...);

CREATE TABLE fk (x timestamp REFERENCES pk (a), ...);

As above, we can probably arrange the operator knowledge to make these
checks.  But I think additionally, you'd need an exclusion constraint on
the PK side to ensure nonoverlapping arrays/periods so that on
update/delete restrict as well as cascading deletes work.
 

Additional interesting examples involve IP network containment using
inet/cidr or ip4/ip4r.  There, you'd probably need additional syntax to
tell the system explicitly which operators to use.