Clemens Ley, François Bry, Tim Furche, Benedikt Linse:
Tim Furche (editor):
Exploring and Taming Existence in Rule-based RDF Queries.

Complete Text [
.pdf, 2.19MB]
In: (I4-D14)

RDF is an emerging knowledge representation formalism proposed by the W3C. A central feature of RDF are blank nodes, which allow to assert the existence of an entity without naming for it. Despite the importance of blank nodes for RDF, many existing RDF query language have only insufficient support for blank nodes. We propose a query language for RDF, called RDFLog, with extensive blank node support. The evaluation of RDFLog may be reduced to the evaluation of Datalog. This allows to apply standard database technology to querying RDF. Our Experimental evaluation shows that our implementation scales well, even for large data sets. The core feature of the reduction is Skolemisation and an new form of un- Skolemisation. In contrast to previous definitions of un-Skolemisation our un-Skolemisation has desirable symmetric properties to those of the Skolemisation. We define a hierarchy of syntactical restrictions of RDFLog with lower expressivity but better complexity, thereby showing the computational costs of blank nodes. We provide both an operational and a denotational semantics of RDFLog and show its soundness and completeness. Finally we discuss a notion of redundancy free answers, called lean answers, and state their complexity and an operational semantics.



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