  Despite the many advantages that ontology-based data access (OBDA) has
brought to a range of application domains, state-of-the-art OBDA systems still
do not support popular graph database management systems such as Neo4j.
Algorithms for query rewriting focus on languages like conjunctive queries and
their unions, which are fragments of first-order logic and were developed for
relational data. Such query languages are poorly suited for querying graph
data. Moreover, they also limit the expressiveness of the ontology languages
that admit rewritings, restricting them to those where the data complexity of
reasoning is not higher than it is in first-order logic. In this paper, we
propose a technique for rewriting a family of navigational queries for a
suitably restricted fragment of ELHI that extends DL-Lite and that is
NL-complete in data complexity. We implemented a proof-of-concept prototype
that rewrites into Cypher queries, and tested it on a real-world cognitive
neuroscience use case with promising results.
