Second-Order Queries for Rule-Based Data Access


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Markus Krötzsch, Sebastian Rudolph

Second-Order Queries for Rule-Based Data Access

Abstract. Rules and ontologies can be used to enrich a database system with advanced data access capabilities. The success of this paradigm has led to a number of languages such as DL-Lite, Datalog+/- and OWL RL. The two major approaches to answering queries under constraints expressed in such languages are forward-chaining (materialization) and backward-chaining (query rewriting). The latter is typically focused on first-order queries that have only limited expressivity. We propose a querying formalism based on monadic second-order logic which subsumes and goes beyond conjunctive queries and regular path queries, but still has a decidable query subsumption problem. We devise methods for rewriting rule sets to queries in this new formalism and we show that query entailment in most of the established rule-based approaches can be decided by combining two methods: (i) bottom-up forward-chaining computation w.r.t. a rule set with the bounded treewidth model property and (ii) top-down second-order query rewriting w.r.t. a rewritable rule set.

Published at Karlsruhe Institute of Technology (Technical report)

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This is an early precursor of the work Flag & Check: Data Access with Monadically Defined Queries. It is suggested to consult the newer version.


Rule languages, Query languages

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