@Override
    public boolean accept(Operator<?> s, Operator<?> t, OperatorGraph.OpEdge opEdge) {
      return ACCEPTABLE_EDGE_TYPES.contains(opEdge.getEdgeType());
    }
To achieve reproducible plans - the operators must be placed in some stable order.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/ParallelEdgeFixer.java