@Override
    public Object process(Node nd, Stack<Node> stack, NodeProcessorCtx procCtx,
        Object... nodeOutputs) throws SemanticException {
      if (nd instanceof ExprNodeConstantDesc) {
        return new NodeInfoWrapper(WalkState.CONSTANT, null,
            (ExprNodeDesc) nd);
      }
      return new NodeInfoWrapper(WalkState.UNKNOWN, null, (ExprNodeDesc)nd);
    }
Processor for constants and null expressions. For such expressions the
processor simply returns.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/pcr/PcrExprProcFactory.java