Query planning with limited source capabilities

In information-integration systems, sources may have diverse and limited query capabilities. We show that because sources have restrictions on retrieving their information, sources not mentioned in a query can contribute to the query result by providing useful bindings. In some cases we can access sources repeatedly to retrieve bindings to answer a query, and query planning thus becomes considerably more challenging. We find all the obtainable answers to a query by translating the query and source descriptions to a simple recursive Datalog program, and evaluating the program on the source relations. This program often accesses sources that are not in the query. Some of these accesses are essential, as they provide bindings that let us query sources, which we could not do otherwise. However, some of these accesses can be proven not to add anything to the query's answer. We show in which cases these off-query accesses are useless, and prove that in these cases we can compute the complete answer to the query by using only the sources in the query. In the cases where off-query accesses are necessary, we propose an algorithm for finding all the useful sources for a query. We thus solve the optimization problem of eliminating the unnecessary source accesses, and optimize the program to answer the query.