The following query is doing a full scan on
If you look at the EXPLAIN of this query, you can see that it visited ALL statements on SPOPredicate(?src, ?p2, ?tgt, ?-
anon-2 then did the (pipeline) join with SPOPredicate(?-sid-1, ?p, ?o, ?-anon-5).
The right way to run this query is to do a scan of the SID region of the SPO index and then unpack the <<?s ?p ?o>> from the subject position of the visited triples. There is no other join.
The RDR syntax <<x y z>> p o implies that <<x y z>> must exist (in a query) and asserts that statement (when parsing data). An interpretation that does not respect this might be the root of the problem.