static String getDataFileName(int hashFileIndex) {
      return String.format(HashTable.OUTPUT_DATA_FILE_PREFIX + "%05d", hashFileIndex);
    }
Choose partitions between row ranges to hash to a single output file Selects region
boundaries that fall within the scan range, and groups them into the desired number of
partitions.
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/HashTable.java