@HiveDecimalWritableVersionV1
  @Override
  public String toString() {
    if (!isSet()) {
      throw new RuntimeException("no value set");
    }
    return fastToString();
  }
Allocate scratchLongs with HiveDecimal.SCRATCH_LONGS_LEN longs.
And, allocate scratch scratchBuffer with HiveDecimal.SCRATCH_BUFFER_LEN_BIG_INTEGER_BYTES bytes.
storage-api/src/java/org/apache/hadoop/hive/serde2/io/HiveDecimalWritable.java