Abstract. Query answering over ontologies is a crucial feature in contexts such as ontology-based data access and semantic information integration. There is considerable research interest in using query rewriting for efficient and scalable query answering: instead of evaluating a given query over the ontology with the (potentially very large) data directly, one rewrites the query with respect to the relevant knowledge in the ontology, and delegates the evaluation of the computed rewriting to a (possibly deductive) database system where the data resides. In this paper we examine the performance and scalability of producing unions of conjunctive queries versus datalog queries as rewritings. We present an empirical comparison between two representative approaches that consider very expressive ontology languages.
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