SEMANTIC QUERY-PROCESSING IN THE VENUS ENVIRONMENT

Recent years have seen the publication of several methodologies for qu ery processing in a Multidatabase (MDB) environment. Whilst different approaches are pertinent to specific CIS architectures, the issue of e fficiency of processing has received relatively little attention. This is somewhat surprising considering the potentially large costs associ ated with MDB queries. In this paper, we consider the efficiency of pr ocessing queries in the VENUS (Esprit 6398) environment. The goal of t his system is to allow a user to query multiple data sources by intera cting with a conceptually single database, and by providing a query in terface based on the use of visual formalisms. This system takes advan tage of existing multidatabase query processing strategies, and deals with optimization issues by considering the exploitation of inter- and intra-schema semantics. We present a methodology which harnesses know ledge of the set-theoretic relationships between classes participating in a query, and demonstrate that significant saving can be made in MD B query processing. The methodology presented here is based on a power ful data model, the Graph Model, which we argue is suitable as the can onical model of a MDB. We also introduce a logic language to formally express interdependencies between classes belonging to different schem as. These assertions form the ''backbone'' of the MDB and, we may bene fit in several ways from the possibility of reasoning about them, spec ifically by applying them in MDB query processing.