@Override
  public TsBlock next() throws Exception {
    if (!hasNext()) {
      throw new NoSuchElementException();
    }
    TsBlock ret = next;
    next = null;
    isBlocked = null;
    return ret;
  }
Try to get next TsBlock. If the next is not ready, return a future. After success, {@link
CountGroupByLevelScanOperator#next} will be set.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/schema/CountGroupByLevelScanOperator.java