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Ontology-based data access (OBDA) is a novel paradigm facilitating access torelational data, realized by linking data sources to an ontology by means ofdeclarative mappings. DL-Lite_R, which is the logic underpinning the W3Contology language OWL 2 QL and the current language of choice for OBDA, hasbeen designed with the goal of delegating query answering to the underlyingdatabase engine, and thus is restricted in expressive power. E.g., it does notallow one to express disjunctive information, and any form of recursion on thedata. The aim of this paper is to overcome these limitations of DL-Lite_R, andextend OBDA to more expressive ontology languages, while still leveraging theunderlying relational technology for query answering. We achieve this byrelying on two well-known mechanisms, namely conservative rewriting andapproximation, but significantly extend their practical impact by bringing intothe picture the mapping, an essential component of OBDA. Specifically, wedevelop techniques to rewrite OBDA specifications with an expressive ontologyto "equivalent" ones with a DL-Lite_R ontology, if possible, and to approximatethem otherwise. We do so by exploiting the high expressive power of the mappinglayer to capture part of the domain semantics of rich ontology languages. Wehave implemented our techniques in the prototype system OntoProx, making use ofthe state-of-the-art OBDA system Ontop and the query answering system Clipper,and we have shown their feasibility and effectiveness with experiments onsynthetic and real-world data.

Beyond OWL 2 QL in OBDA: Rewritings and Approximations (Extended Version)