
161Large-Scale RDF Processing with MapReduce
applied to the default graph. The graph operator can be used to apply a pattern to
one or all of the named graphs. A named graph is referenced by an unique URI,
and for each graph that is used in the query, we need a pair (URI, graph) that speci-
es where to nd the corresponding RDF graph. If a variable is used in the Graph
operator instead of a specic graph URI, the pattern must be applied to all named
graphs.
As we want to execute SPARQL queries on large RDF graphs in a MapReduce
cluster, all graphs must be stored in the distributed le system. Applying a pattern
to one of the named graphs with ...