The Two Views on Ontological Query Answering

Abstract. We attempt a structured view at the ontological query answering prob-lem by distinguishing between two antagonistic perspectives: The knowledge rep-resentation perspective considers the ontology as a part of the specified knowl-edge whereas the database perspective assumes it to be part of the query. These two perspectives give rise to two computation strategies: (data-driven) forward chaining and (query-driven) backward chaining, based on which different types of decidability criteria can be defined. We give an overview of the two views as well as ensuing conditions for decidability, focusing on existential rules as onto-logical formalism that has lately gained a lot of renewed interest from both the KR and DB communities.