Query Inseparability for Description Logic Knowledge Bases

ABSTRACT. We investigate conjunctive query inseparability of description logic (DL) knowledge bases (KBs) w.r.t. a given signature, a fundamental problem for KB versioning, module extraction, forgetting and knowledge exchange. We study the data and combined complexity of deciding KB query inseparability for fragments of Horn-ALCHI, including the DLs underpinning OWL 2 QL and OWL 2 EL. While all of these DLs turn out to be P-complete for data complexity, the combined complexity ranges from P to EXPTIME and 2EXPTIME. We also resolve two major open problems for OWL 2 QL by showing that TBox query inseparability and the membership problem for universal UCQ solutions in knowledge exchange are both EXPTIME-complete for combined complexity.