public static RecordReader createReaderFromFile(Reader file,
                                                  Configuration conf,
                                                  long offset, long length
  ) throws IOException {
    return createReaderFromFile(file, conf, offset, length, isOriginal(file));
  }
Get the root column for the row. In ACID format files, it is offset by
the extra metadata columns.
@param isOriginal is the file in the original format?
@return the column number for the root of row.
ql/src/java/org/apache/hadoop/hive/ql/io/orc/OrcInputFormat.java