Do foreign keys have size?

Question

I am calculating a database size and I had one doubt about foreign keys. Do they have size? If they are references, do they occupy two different positions in memory?

Answer 1

The columns that make up the foreign key do, of course, have size, but they would occupy the same amount of space whether or not they are participating in any foreign key relationship.

Most database products require that the target column(s) of a foreign key reference by indexed - but, again, such an index can exist even in the absence of a foreign key relationship.

Thirdly, the foreign key will require a small (fixed) amount of metadata to record its existence.

Beyond that, there may be some database products that do require some additional space to implement some foreign key features, such as cascading deletes. Such details would be product specific.

But in general, for plain foreign keys, there should be no additional storage requirement.