How references differ from primary keys?
Primary key is a normal column adapted to special purposes. Reference is an initially special mechanism, which has much less to do with object properties. In other words, reference is NOT a column or dimension. It is a separable part of an object, which is intended to represent the source object in another space. In fact, reference may have properties (both its own and object’s) but the difference from primary key is in the role of the identifier.
