public static List<String> getQualifedReducerKeyNames(List<String> keyExprs) {
    List<String> result = Lists.newArrayList();
    if (keyExprs != null) {
      for (String key : keyExprs) {
        result.add(Utilities.ReduceField.KEY + "." + key);
      }
    }
    return result;
  }
Get qualified column name from output key column names.
@param keyExprs
         - output key names
@return list of qualified names
ql/src/java/org/apache/hadoop/hive/ql/stats/StatsUtils.java