Type-checking OQL queries in the ODMG type systems

Abstract

Several negative results are proved about the ability to type-check queries in the only existing proposed standard for object-oriented databases. The first of these negative results is that it is not possible to type-check OQL queries in the type system underlying the ODMG object model and its definition language ODL. The second negative result is that OQL queries cannot be type-checked in the type system of the Java binding of the ODMG standard either. A solution proposed in this paper is to extend the ODMG object model with explicit support for parametric polymorphism (universal type quantification). These results show that Java cannot be a viable database programming language unless extended with parametric polymorphism. This is why type-checking OQL queries presents no problem for the type system of the C++ binding of the ODMG standard. However, a type system that is strictly more powerful than any of the type systems of the ODMG standard is required in order to properly type ordered collectgions and indices. The required form of polymorphism is bounded type quantification (constrained genericity) and even F-bounded polymorphism. A further result is that neither static nor the standard dynamic object-oriented type-checking is possible for Java OQL, in spite of the fact that Java OQL combines features of two strongly and mostly statically-typed languages. Contrary to one of the promises of object-oriented database technology, this result shows that the impedance mismatch does not disappear in the ODMG standard. A type-safe reflective technique is proposed for overcoming this mismatch.