| Abstract: | Views are an important functionality provided by the relational database systems. However, commercial object-oriented database systems do not support a view mechanism because defining the semantics of views in the context of an object-oriented model is more difficult than in the relational model. Indeed, views are not included in the ODMG standard. In this paper, we present a proposal aimed at including views in the ODMG, by extending the object model and the object definition language (ODL). We consider object-oriented views as having the same functionality as relational views. Views are included in the object model in such a way that (i) views make a new kind of data type definition, just as are classes, interfaces and literals, (ii) an IS-VIEW relationship is introduced in order to specify the derivation of a view from its base class, and (iii) a view instance preserves the identity of its base instance. A view can import attributes, relationships and operations from its base class, and it can also add new operations, derived attributes and derived relationships. The extent of the view is defined by an object query language (OQL) predicate. We also describe a C++ binding showing the practicability of the proposed model. |
