static RelNode stripHep(RelNode rel) {
    if (rel instanceof HepRelVertex) {
      HepRelVertex hepRelVertex = (HepRelVertex) rel;
      rel = hepRelVertex.getCurrentRel();
    }
    return rel;
  }
Registers a relational expression and the relational expression it became
after decorrelation.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/rules/HiveRelDecorrelator.java