Games for query inseparability of description logic knowledge bases

We consider conjunctive query inseparability of description logic knowledge bases with respect to a given signature-a fundamental problem in knowledge base versioning, module extraction, forgetting and knowledge exchange. We give a uniform game-theoretic characterisation of knowledge base conjunctive query inseparability and develop worst-case optimal decision algorithms for fragments of Horn-ALCHI, including the description logics underpinning OWL2 QL and OWL2 EL. We also determine the data and combined complexity of deciding query inseparability. While query inseparability for all of these logics is P-complete for data complexity, the combined complexity ranges from P- to EXPTIME- to 2EXPTIME-completeness. We use these results to resolve two major open problems for OWL2 QL by showing that TBox query inseparability and the membership problem for universal conjunctive query solutions in knowledge exchange are both EXPTIME-complete for combined complexity. Finally, we introduce a more flexible notion of inseparability which compares answers to conjunctive queries in a given signature over a given set of individuals. In this case, checking query inseparability becomes NP-complete for data complexity, but the EXPTIME- and 2EXPTIME-completeness combined complexity results are preserved. (C) 2016 Elsevier B.V. All rights reserved.