Abstract
We investigate conjunctive query inseparability of description logic knowledge bases (KBs) with respect to a given signature, a fundamental problem for KB versioning, module extraction, forgetting and knowledge exchange. We develop a game-theoretic technique for checking query inseparability of KBs expressed in fragments of Horn-ALCHI, and show a number of complexity results ranging from P to EXPTIME and 2EXPTIME. We also employ our results to resolve two major open problems for OWL 2 QL by showing that TBox query in-separability and the membership problem for universal UCQ-solutions in knowledge exchange are both EXPTIME-complete for combined complexity.