Semantics and Complexity of SPARQL

SPARQL is the W3C candidate recommendation query language for RDF. In this paper the authors address systematically the formal study of SPARQL, concentrating in its graph pattern facility. They consider for this study a fragment without literals and a simple version of filters which encompasses all the main issues yet is simple to formalize. They provide a compositional semantics, prove there are normal forms, prove complexity bounds, among others that the evaluation of SPARQL patterns is PSPACE-complete, compare the semantics to an alternative operational semantics, give simple and natural conditions when both semantics coincide and discuss optimizations procedures.