Obtain statistics on the RDF database. Defined statistics are:
Number of named graphs.
Total number of triples in the database. This is the number
of asserted triples minus the number of retracted ones. The
number of visible triples in a particular context may be
different due to visibility rules defined by the logical
update view and transaction isolation.
Number of resources that appear as subject or object in a
triple. See rdf_resource/1.