Efficient Ontological Query Answering by Rewriting into Graph Queries

The OWL 2 QL profile of the OWL 2 Web Ontology Language, based on the family of description logics called DL-Lite, allows for answering queries by rewriting, i.e. by reformulating a given query into another query that is then directly processed by a RDBMS system by pure querying, without materialising new data or updating existing data. In this paper we propose a new language whose expressive power goes beyond that of DL-Lite (in particular, our language extends both OWL 2 QL and linear \(\mathcal {ELH}\), two well known DL ontology languages) while still allowing query answering via rewriting of queries into conjunctive two-way regular path queries (C2RPQs). Our language is identified by a syntactic property that can be efficiently checked. After defining our new language, we propose a novel rewriting technique for conjunctive queries (CQs) that makes use of nondeterministic finite state automata. CQ answering in our setting is NLogSpace-complete in data complexity and NP-complete in combined complexity; answering instance queries is NLogSpace-complete in data complexity and in \(\textsc {PTime}\) in combined complexity.