Answering recursive queries using views

We consider the problem of answering datalog queries using materialized views. The ability to answer queries using views is crucial in the context of information integration. Previous work on answering queries using views restricted queries to being conjunctive. We extend this work to general recursive queries: Given a datalog program P and a set of views, is it possible to nd a datalog program that is equivalent toPand only uses views as EDB predicates? In this paper, we show that the problem of whether a datalog program can be rewritten into an equivalent program that only uses views is undecidable. On the other hand, we prove that a datalog program P can be e ectively rewritten into a program that only uses views, that is contained in P, and that contains all programs that only use views and are contained in P. As a consequence, if there exists a program equivalent toPthat only uses views, then our construction is guaranteed to yield a program equivalent toP.