document: Rewriting queries on SPARQL views The problem of answering SPARQL queries over virtual SPARQL views is commonly encountered in a number of settings , including while enforcing security policies to access RDF data , or when integrating RDF data from disparate sources . We approach this problem by rewriting SPARQL queries over the views to equivalent queries over the underlying RDF data , thus avoiding the costs entailed by view materialization and maintenance . We show that SPARQL query rewriting combines the most challenging aspects of rewriting for the relational and XML cases : like the relational case , SPARQL query rewriting requires synthesizing multiple views ; like the XML case , the size of the rewritten query is exponential to the size of the query and the views . In this paper , we present the first native query rewriting algorithm for SPARQL . For an input SPARQL query over a set of virtual SPARQL views , the rewritten query resembles a union of conjunctive queries and can be of exponential size . We propose optimizations over the basic rewriting algorithm to ( i ) minimize each conjunctive query in the union ; ( ii ) eliminate conjunctive queries with empty results from evaluation ; and ( iii ) efficiently prune out big portions of the search space of empty rewritings . The experiments , performed on two RDF stores , show that our algorithms are scalable and independent of the underlying RDF stores . Furthermore , our optimizations have order of magnitude improvements over the basic rewriting algorithm in both the rewriting size and evaluation time .
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