Integration of Maximum Information Using Outerjoins, Predicates and Foreign Functions

The goal of this paper is to present algorithms for creating an optimized query plan for retrieving maximum information from multiple relations, using outerjoins. Especially we focus on conjunctive queries in the presence of predicates and foreign functions. We show first with examples that retrieving maximum information by integrating multiple relations requires outerjoin operators. The outerjoin is essential to prevent information loss that would be caused by the inner join. We also show that predicates and foreign functions are useful to mediate the discrepancy among the relations and to create arbitrary views. Outerjoins and foreign functions, together with predicates, make it difficult to create query processing plans since they impose restrictions on the order of query processing. The rest of this paper describes algorithms for creating such query processing plans for conjunctive queries expressed in extended Datalog. First, we show simple algorithms for creating query plans with outerjoins, but without predicates and foreign functions. We use the hypergraph representation of the relations to explain an optimized algorithm. Then, we show a more complex algorithm that works for query plans with predicates and foreign functions. In our algorithm, we create an initial expression graph whose nodes represent query processing units, including outerjoin, predicate and foreign function operators. Then, we convert the initial expression graph into an executable, optimized expression tree. This algorithm is implemented and deployed in a mediation system that integrates heterogeneous information sources. key words: database, query processing, outerjoin, integration