Ontological Query Answering via Rewriting

Ontological databases extend traditional databases with ontological constraints. This technology is crucial for many applications such as semantic data publishing and integration as well as model-driven database design. For many classes of ontological constraints, query answering can be solved via query rewriting. In particular, given a conjunctive query and a set of ontological constraints, the query is compiled into a first-order query, called the perfect rewriting, that encodes the intensional knowledge implied by the constraints. Then, for every database D, the answer is obtained by directly evaluating the perfect rewriting over D. Since first-order queries can be easily translated into SQL, ontological query answering can be delegated to traditional DBMSs. This allows us to utilize all the query optimization techniques available in the underlying DBMS. This paper surveys current approaches to rewriting-based query answering of ontological databases.