November 10,2016, 10:45 am, Stanislav Kikot (Birkbeck College, University of London)
November 10,2016, 10:45 am, Room 02/022, Building 5, Saint Priest – Stanislav Kikot (Birkbeck College, University of London)
Title: The Complexity of Ontology-Based Data Access with OWL 2 QL and Bounded Treewidth Queries
Abstract: Our concern is the overhead of answering OWL 2 QL ontology-mediated queries (OMQs) in ontology-based data access compared to evaluating their underlying tree-shaped and bounded treewidth conjunctive queries (CQs). We will demonstrate that OMQs with bounded-depth ontologies have nonrecursive datalog (NDL) rewritings that can be constructed and evaluated in LOGCFL for combined complexity, even in NL if their CQs are tree-shaped with a bounded number of leaves, and so incur no overhead in complexity-theoretic terms. For OMQs with arbitrary ontologies and bounded-leaf CQs, NDL-rewritings are constructed and evaluated in LOGCFL. We will also present experimental results suggesting feasibility and scalability of our rewritings compared to standard NDL-rewritings. On the negative side, we prove that answering OMQs with tree-shaped CQs is not fixed-parameter tractable if the ontology depth or the number of leaves in the CQs is regarded as the parameter, and that answering OMQs with a fixed ontology (of infinite depth) is NP-complete for tree-shaped and LOGCFL for bounded-leaf CQs.