On the Role of the GRAPH Clause in the Performance of Federated SPARQL Queries

Federated SPARQL queries give unified answers from multiple and distributed SPARQL endpoints. A good example may be the collection of stops from different transport companies in the same city to create a route planning application. The performance of the evaluation of these types of queries is usually poor, a fact that makes difficult their use in real-life applications that need good performance requirements. In this paper we present a preliminary analysis on the improvement that can be achieved by using the GRAPH clause in federated SPARQL queries. The main goal is to reduce the search space of such queries by setting the NAMED GRAPH to the graph pattern where the corresponding patterns should be evaluated. We perform a preliminary comparison between a federated query and a rewriting that uses systematically the GRAPH clause. These experiments show that the inclusion of the GRAPH clause may only improve performance of query evaluation between 5% and 10%. These early findings suggest that, although the GRAPH clause may indeed play a role in speeding up federated SPARQL queries, hurdles are yet to be overcome when using the GRAPH clause as named graphs are semantically ambiguous