Evaluating SPARQL Subqueries over P2P Overlay Networks

The introduction of subqueries is one of the most interesting features included in the latest SPARQL 1.1 specification. Existing distributed RDF storage and querying systems have not studied the evaluation of this newly included query feature. The evaluation of subqueries in distributed environment may be very inefficient and expensive in terms of query response time and bandwidth usage, particularly for the correlated subqueries where the inner query block is evaluated once for each solution of the outer query. In this paper, we study the problem of evaluating SPARQL subqueries over RDF data stored in 3nuts p2p network. We study semi join based optimization technique, and transformation algorithms to transform correlated queries to equivalent uncorrelated ones, that would make the distributed evaluation of such queries more efficient.