@Override
  public boolean hasNext() throws Exception {
    isBlocked().get(); // wait for the next TsBlock
    if (!schemaReader.isSuccess()) {
      throw new SchemaExecutionException(schemaReader.getFailure());
    }
    return next != null;
  }
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