Subtyping by Constraints in Object-Oriented Databases

For many object-oriented database applications taxonomies with a setinclusion semantics among the type extents are essential. In practical cases, however, common object-oriented modeling techniques often do not result in taxonomies as they ignore application specific constraints. We will elaborate that especially in domains like CAD or similar engineering environments integrity constraints on type attributes have a deep impact on the resulting hierarchy. We argue that subtyping by constraints may be superior to other object-oriented alternatives like subtyping for generalization or nearly-flat hierarchies. Subtyping by constraints achieves a logical set-inclusion hierarchy, and in addition enables a larger amount of semantically correct substitutability. This can even be improved by a novel framework of automatic method adaptation for enhanced substitutability. Moreover, a potential storage penalty caused by making constraints explicit can be avoided completely by applying a new storage optimization technique based on functional integrity constraints. Our results are illustrated by practical examples drawn from the OCAD project.?