private SemanticNodeProcessor getDefaultProc() {
    return new SemanticNodeProcessor() {
      @Override
      public Object process(Node nd, Stack<Node> stack,
                            NodeProcessorCtx procCtx, Object... nodeOutputs)
          throws SemanticException {
        return null;
      }
    };
  }
this transformation does bucket map join optimization.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/BucketMapJoinOptimizer.java