@Override
  public long calculateMaxReturnSize() {
    // the long used for count
    return 8L;
  }
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