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