Enhancing OODB semantics to support browsing in an OODB vacabulary representation

In previous work, we have modeled a vocabulary given as a semantic network by an object-oriented database (OODB). The OODB schema thus obtained provides a compact abstract view of the vocabulary. This enables the fast transversal of the vocabulary by a user. In the semantic network vocabulary, the IS-A relationships express the specialization hierarchy. In our OODB modeling of the vocabulary, the SUBCLASS relationship expresses the specialization hierarchy of the classes and supports the inheritance of their properties. A typical IS-A path in the vocabulary has a corresponding shorter SUBCLASS path in the OODB schema. In this paper we expose several cases where the SUBCLASS hierarchy fails to fully correspond to the IS-A hierarchy of the vocabulary. In these cases there exist transversal paths in the semantic network for which there are no corresponding transversal paths in the OODB schema. The reason for this failure is the existence of some IS-A relationships between concepts of two classes, which are not connected by a SUBCLASS relationship. This phenomenon weakens the accuracy of our modeling. To rectify the situation we introduce a new OODB semantic relationship IS-A′ to represent the existence of IS-A relationships between concepts of a pair of classes which are not connected via a SUBCLASS relationship. The resulting schema contains both SUBCLASS relationships and IS-A′ relationships which completely model the IS-A hierarchy of the vocabulary. We define a mixed-class level transversal path to contain either SUBCLASS or IS-A′ relationships. Consequently, each transversal path in the semantic network has a corresponding mixed transversal path in the OODB schema. Hence the introduction of the semantic OODB IS-A′ relationship improves the modeling of semantic network vocabularies by OODBs.