private static String toReadColumnIDString(List<Integer> ids) {
    String id = "";
    for (int i = 0; i < ids.size(); i++) {
      if (i == 0) {
        id = id + ids.get(i);
      } else {
        id = id + StringUtils.COMMA_STR + ids.get(i);
      }
    }
    return id;
  }
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