Аннотация
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-LiteR, 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-LiteR, 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-LiteR 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.
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