Department of Computer, Control and Management Engineering

Hi(OWL 2 QL) is a new ontology language with the OWL2QL syntax and a specific semantics designed to support metamodeling and metaquerying. In this paper we investigate the problem of answering metaqueries in Hi(OWL 2 QL), which are unions of conjunctive queries with both ABox and TBox atoms. We first focus on a specific class of ontologies, called TBox-complete, where there is no uncertainty about TBox axioms, and show that query answering in this case has the same complexity (both data and combined) as in OWL 2 QL. We then move to general ontologies and show that answering metaqueries is coNP-complete with respect to ontology complexity, Π2p-complete with respect to combined complexity, and remains AC0 with respect to ABox complexity. Finally, we present an optimized query answering algorithm that can be used for TBox-complete ontologies.