Horn-Rewritability vs PTime Query Evaluation in Ontology-Mediated Querying.

© 2018 International Joint Conferences on Artificial Intelligence. All right reserved. In ontology-mediated querying with an expressive description logic (DL) L, two desirable properties of a TBox T are (1) being able to replace T with a TBox formulated in the Horn-fragment of L without affecting the answers to conjunctive queries (CQs) and (2) that every CQ can be evaluated in PTIME w.r.t. T . We investigate in which cases (1) and (2) are equivalent, finding that the answer depends on whether the unique name assumption (UNA) is made, on the DL under consideration, and on the nesting depth of quantifiers in the TBox. We also clarify the relation between query evaluation with and without UNA and consider natural variations of property (1).