Using semiouterjoins to process queries in multidatabase systems

A multidatabase system provides a logically integrated view of existing, possibly inconsistent, databases. Logical integration is achieved primarily through the use of generalization, which can be modelled algebraically as a sequence of outerjoin and aggregation operations. Conventional distributed query processing techniques are inadequate for processing queries over views defined by outerjoins and aggregates. In a conventional distributed database system, selections and projections are inexpensive to process; hence joins have been the rocus of most previous research. In a multidatabase system, however, even selections and projections can be as expensive as joins. The semiouterjoin operation can potentially reduce query processing costs. In general, there may be many different strategies based on semiouterjoins for processing a given query. The query optimization problem is to choose the most profitable of these strategies. This paper studies the query optimization problem for selection and projection queries. It develops linear-time solutions to the problem, and then extends these solutions to provide heuristics for joins and conjunctive queries.