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