@Override
  public Split getNext(int subtaskId) {
    return getNextSplit(subtaskId)
        .map(Split::of)
        .orElseGet(isEmpty() ? Split::unavailable : Split::subtaskUnavailable);
  }
Key is subtaskId, Value is the queue of unAssigned mixed-format splits.
amoro-format-mixed/amoro-mixed-flink/amoro-mixed-flink-common/src/main/java/org/apache/amoro/flink/read/hybrid/assigner/ShuffleSplitAssigner.java