The design of the POSTGRES data model is described. The data model is a relational model that has been extended with abstract data types including user-defined operators and procedures, relation attributes of type procedure, and attribute and procedure inheritance. These mechanism can be used to simulate a wide variety of semantic and object-oriented data modeling constructs including aggregation and generalization, complex objects with shared subobjects, and attributes that reference tuples in other relations.
Lawrence A. Rowe, Michael Stonebraker