public static void appendNestedColumnPaths(
    Configuration conf,
    List<String> paths) {
    if (paths == null || paths.isEmpty()) {
      return;
    }
    String pathsStr = StringUtils.join(StringUtils.COMMA_STR,
      paths.toArray(new String[paths.size()]));
    String old = conf.get(READ_NESTED_COLUMN_PATH_CONF_STR, null);
    String newConfStr = pathsStr;
    if (old != null && !old.isEmpty()) {
      newConfStr = newConfStr + StringUtils.COMMA_STR + old;
    }
    setReadNestedColumnPathConf(conf, newConfStr);
  }
Appends read nested column's paths. Once a read nested column path
is included in the list, a underlying record reader of a columnar file format
(e.g. Parquet and ORC) can know what columns are needed.
serde/src/java/org/apache/hadoop/hive/serde2/ColumnProjectionUtils.java