private static void appendReadColumnNames(Configuration conf, List<String> cols) {
    String old = conf.get(READ_COLUMN_NAMES_CONF_STR, "");
    String delim = conf.get(serdeConstants.COLUMN_NAME_DELIMITER, String.valueOf(SerDeUtils.COMMA));
    String result = String.join(delim, cols);
    if (!old.isEmpty()) {
      result = old + delim + result;
    }
    conf.set(READ_COLUMN_NAMES_CONF_STR, result);
  }
Returns an array of column ids(start from zero) which is set in the given
parameter <pre>conf</pre>.
serde/src/java/org/apache/hadoop/hive/serde2/ColumnProjectionUtils.java