Treffer: On Type Systems for Object-Oriented Database Programming Languages

properties in the ODMG Object Model include abstract state and abstract relationships (two-way mappings). Relationships can only be defined between classes. Abstract behavior is specified as a set of operations. Behavior specifications are novariant in both their argument and return types. The ODMG Object Model supports single dispatch. Several system-defined parametric container classes are present in the object model, but the user is not allowed to define new ones. Type parameters can only be used in property specifications; operation specifications can not be parameterized. OQL is a strongly typed query language that provides a possibility of explicit dynamically checked type conversions. Set operations in OQL can only be performed on "compatible types" (types that have the least upper bound). Since the notion of the greatest lower bound is not available in OQL, all set operations use the least upper bound for typing purposes. For example, intersecting a set of students with a set of persons would return a result of type "set of persons" even in the case when the type of students is a subtype of the person type. ODMG/OQL fails all tests except for BROWSER and the union part of the SET test. It should be noted, however, that ODMG/OQL is not a general-purpose database programming language, and therefore its performance on the test suite is not fully indicative of the merits of the ODMG Object Model.