Section: New Results

Selectivity Estimation for SPARQL Triple Patterns with Shape Expressions for Optimising SPARQL Query Evaluation

ShEx (Shape Expressions) is a language for expressing constraints on RDF graphs. In this work [11], [8], [15], we optimize the evaluation of conjunctive SPARQL queries, on RDF graphs, by taking advantage of ShEx constraints. Our optimization is based on computing and assigning ranks to query triple patterns, dictating their order of execution. The presence of intermediate joins between the query triple patterns is the reason why ordering is important in increasing efficiency. We first define a set of well formed ShEx schemas, that possess interesting characteristics for SPARQL query optimization. We then define our optimization method by exploiting information extracted from a ShEx schema. We finally report on evaluation results performed showing the advantages of applying our optimization on the top of an existing state-of-the-art query evaluation system.