Does a Foreign Key referencing PK need the NOT NULL constraint?

Question

Does a Foreign Key referencing a Primary Key need the NOT NULL constraint in a PostgreSQL database?
The database is highly normalized and will be very large. I do not wish to add extra constraints that will slow down the queries even more if said queries are unneeded.

Answer 1

A primary key must be unique, and ideally it should be picked by the database, to limit problems with concurrency, so, though it can be unique, for 1 record, every other record must have a value.

Otherwise, how will you know which row this foreign key relates to if there is more than one match?

So, as New in town mentioned, NULL should be valid, but for one record, as the uniqueness will be the larger issue.

EDIT: oops, misunderstood the question.

I have put null on foreign keys before, but if you have cascade delete for example then null won't work, unless you have the key in the primary table with a null value.

Answer 2

If you want to be able to represent unknown in the FK column of that table, then make it nullable, if it has to have a value, make it Not Null.

You can have as many records as you want in the referencing table with null FK values. The unique constraint is on the rows in the referenced table (Where the PK is) not on the rows in the referencing table (where the FK is).