We study fundamental aspects related to the efficient processing of the SPARQL query language for RDF, proposed by the W3C to encode machine-readable information in the Semantic Web. Our key contributions are (i) a complete complexity analysis for all operator fragments of the SPARQL query language, which -as a central result -shows that the SPARQL operator OPTIONAL alone is responsible for the PSPACE-completeness of the evaluation problem, (ii) a study of equivalences over SPARQL algebra, including both rewriting rules like filter and projection pushing that are wellknown from relational algebra optimization as well as SPARQLspecific rewriting schemes, and (iii) an approach to the semantic optimization of SPARQL queries, built on top of the classical chase algorithm. While studied in the context of a theoretically motivated set semantics, almost all results carry over to the official, bag-based semantics and therefore are of immediate practical relevance.
Finally, Schmidt et al. REF study the set of equivalences over the SPARQL algebra as well as well known to relational algebra rewriting rules.
Foundations of SPARQL query optimization
ICDT '10
Computer Science