Schema Versions and Dag Rearrangement Views in Object-oriented Databases

An important requirement of non-traditional database applications such as computer aided design, artificial intelligence, and office information systems with multimedia documents is the support of application evolution of object schemas as well as evolution of objects in the application. We provided a framework of schema evolution in [BKKK86, BKKK87] and the framework was realized in an object-oriented database system, ORION, at MCC. In this paper, we extend this schema evolution framework by allowing schema versions and DAG rearrangement views in object-oriented databases. We present a technique that enables users to manipulate schema versions explicitly and maintain schema evolution histories. For completeness, we integrate our model with the object version model formulated by H.T. Chou and W. Kim [CK86]. We identify new types of view, called DAG rearraangement views, of composite objects and class hierarchies. We present a set of operators for defining DAG rearrangement views. We identify sets of composite object views with the property that queries on the views are processable on instances of the original composite object schema. We also discuss how instances would be viewed and reorganized in DAG rearrangement views of class hierarchies.