Using Heterogeneous Equivalences for Query Rewriting in Multidatabase Systems

In order to have significant practical impact on future information systems, multidatabase management systems (MDBMS) must be both flexible and efficient. We consider a MDBMS with a common object-oriented model, based on the ODMG standard, and local databases that may be relational or object-oriented. In this context, query rewriting (for optimization) is made difficult by schematic discrepancy, and the need to model mapping information between the multidatabase and local schemas. We address the flexibility issue by representing the mappings from a local schema to the multidatabase schema, as a set of heterogeneous object equivalences, in a declarative language. Efficiency is obtained by exploiting these equivalences to rewrite multidatabase OQL queries into equivalent, simplified queries on the local schemas. 1 Introduction The advent of open systems is increasingly stimulating the development of information systems which can provide high-level integration of heterogeneous informatio..