@Override
    public void onMatch(RelOptRuleCall call) {
      final HepRelVertex rel = call.rel(0);
      call.transformTo(rel.getCurrentRel());
    }
This rule is used within the PROGRAM that rewrites the query for
partial rewritings. Its goal is to extract the RelNode from the
HepRelVertex node so the rest of the rules in the PROGRAM can be
applied correctly.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/views/HiveMaterializedViewRule.java