@Override
    public int compare(Pair<Operator<?>, Operator<?>> o1, Pair<Operator<?>, Operator<?>> o2) {
      return sig(o1).compareTo(sig(o2));
    }
To achieve reproducible plans - the operators must be placed in some stable order.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/ParallelEdgeFixer.java