Object Views and Database Restructuring

We present a formal data model for views in Object DataBase Systems (ODBS) as a transformation mechanism for databases. Our model relaxes the usual constraint where an object belongs to a single class while using a generalization of referent and enables a deterministic creation of derived objects with complex object identiiers. We deene an IQL-like language which enables the manipulation of such referents. The view-based transformation is achieved in two steps: an extension of the source base followed by a projection of the extended base. The extension and projection can be carried out using four object algebraic operators, namely projection, join-specialization, join and generalization, that specify both the virtual schema and its corresponding virtual instance. This simple algebra can express most the view operators proposed in the literature and provides a real restructing of the source schema and base.