In this paper we present Prexto, an algorithm for computing a perfect rewriting of unions of conjunctive queries over ontologies expressed in the description logic DL-Lite_a- The main novelty of Prexto lies in the fact that it constitutes the first technique for query rewriting over ontologies which fully exploits extensional constraints to optimize query rewriting. In addition, Prexto makes use of functional role axioms and of concept and role disjointness axioms to optimize the size of the rewritten query. We show that these optimizations allow Prexto to outperform the existing query rewriting techniques for DL-Lite in practical cases.
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