@Override
  public ExprNodeDesc clone() {
    return new ExprNodeConstantDesc(typeInfo, value);
  }
Return string representation of constant expression
Beside ExplainPlan task Default constraint also make use it to deserialize constant expression
to store it in metastore, which is later reparsed to generate appropriate constant expression
Therefore it is necessary for this method to qualify the intervals with appropriate qualifiers
ql/src/java/org/apache/hadoop/hive/ql/plan/ExprNodeConstantDesc.java