@HiveDecimalWritableVersionV2
  public boolean isValid() {
    if (!isSet) {
      return false;
    }
    return FastHiveDecimalImpl.fastIsValid(this);
  }
See the comments for HiveDecimal.serializationUtilsWrite.
storage-api/src/java/org/apache/hadoop/hive/serde2/io/HiveDecimalWritable.java