Simulation Subsumption or Déjà vu on the Web

Authors:
François Bry;Tim Furche;Benedikt Linse
Affiliations:
Institute for Informatics, University of Munich, München, Germany 80538;Institute for Informatics, University of Munich, München, Germany 80538;Institute for Informatics, University of Munich, München, Germany 80538
Venue:
RR '08 Proceedings of the 2nd International Conference on Web Reasoning and Rule Systems
Year:
2008

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Abstract

Simulation unification is a special kind of unification adapted to retrieving semi-structured data on the Web. This article introduces simulation subsumption, or containment, that is, query subsumption under simulation unification. Simulation subsumption is crucial in general for query optimization, in particular for optimizing pattern-based search engines, and for the termination of recursive rule-based web languages such as the XML and RDF query language Xcerpt. This paper first motivates and formalizes simulation subsumption. Then, it establishes decidability of simulation subsumption for advanced query patterns featuring descendant constructs, regular expressions, negative subterms (or subterm exclusions), and multiple variable occurrences. Finally, we show that subsumption between two query terms can be decided in $\mathcal{O}(n!^n)$ where n is the sum of the sizes of both query terms.