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Séminaire d'équipe(s)
Goal-oriented Query Rewriting for OWL 2 QL
Despoina Trivela

23 November 2012, 14h00 - 23 November 2012, 15h00
Salle/Bat : 455/PCRI-N
Contact : Brigitte Safar (Seminaire Equipe IASI)

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Résumé :
The use of ontologies in data access allows for semantic query answering, i.e. for answering user queries expressed in terms of terminologies linked to the data. Queries are typically expressed in the form of conjunctive queries (CQ) and terminologies in an ontological form.
Unfortunately, the problem of answering CQs in terms of ontologies axiomatized in expressive Description Logics (DL) suffers from high worst-case complexity.
The DL-Lite$_R$ language, which underpins the OWL 2 QL profile, overcomes this problem by allowing a limited expressivity. In DL-Lite$_R$, the CQ answering problem is tractable from the data point of view, and can be solved by splitting the answering procedure in two steps: the query rewriting, in which the CQ is expanded into a union of CQs (UCQ), and the execution of the UCQ over the database. Apart from having the advantage of using the mature relational database technology, rewriting can be based on first order resolution-based reasoning algorithms. The main restriction is that, for large terminologies and/or large queries, the exponential complexity in the query size may result in a very large number of rewritings. We present Rapid, a query rewriting algorithm for queries posed over DL-Lite$_R$ ontologies. Instead of exhaustively performing resolution, it performs a restricted sequence of inferences that lead directly to rewriting sets with less redundant rewritings. In this way, we avoid a large number of blind inference paths and the need for extended query subsumption checks.

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