Beyond OWL 2 QL in OBDA: Rewritings and Approximations (Extended
  Version)
  Ontology-based data access (OBDA) is a novel paradigm facilitating access to
relational data, realized by linking data sources to an ontology by means of
declarative mappings. DL-Lite_R, which is the logic underpinning the W3C
ontology language OWL 2 QL and the current language of choice for OBDA, has
been designed with the goal of delegating query answering to the underlying
database engine, and thus is restricted in expressive power. E.g., it does not
allow one to express disjunctive information, and any form of recursion on the
data. The aim of this paper is to overcome these limitations of DL-Lite_R, and
extend OBDA to more expressive ontology languages, while still leveraging the
underlying relational technology for query answering. We achieve this by
relying on two well-known mechanisms, namely conservative rewriting and
approximation, but significantly extend their practical impact by bringing into
the picture the mapping, an essential component of OBDA. Specifically, we
develop techniques to rewrite OBDA specifications with an expressive ontology
to "equivalent" ones with a DL-Lite_R ontology, if possible, and to approximate
them otherwise. We do so by exploiting the high expressive power of the mapping
layer to capture part of the domain semantics of rich ontology languages. We
have implemented our techniques in the prototype system OntoProx, making use of
the state-of-the-art OBDA system Ontop and the query answering system Clipper,
and we have shown their feasibility and effectiveness with experiments on
synthetic and real-world data.
