@HiveDecimalWritableVersionV2
  public void deserialize64(
      long decimal64Long, int scale) {
    fastDeserialize64(decimal64Long, scale);
    isSet = true;
  }
See the comments for HiveDecimal.serializationUtilsWrite.
storage-api/src/java/org/apache/hadoop/hive/serde2/io/HiveDecimalWritable.java