pgsql-bugs(at)postgresql(dot)org writes:
> If you have a primary key on several columns, each of these columns is
> given the 'not null' modifier. I can't see why this is required.
(A) it makes no sense otherwise, and (B) the SQL spec says so:
A unique constraint is satisfied if and only if no two rows in
a table have the same non-null values in the unique columns. In
addition, if the unique constraint was defined with PRIMARY KEY,
then it requires that none of the values in the specified column or
columns be the null value.
If you don't want NOT NULL, maybe what you are after is a plain UNIQUE
constraint, not PRIMARY KEY.
regards, tom lane