private TsBlock constructTsBlockAndClearMap(Map<PartialPath, Long> countMap) {
    TsBlockBuilder tsBlockBuilder = new TsBlockBuilder(OUTPUT_DATA_TYPES);
    for (Map.Entry<PartialPath, Long> entry : countMap.entrySet()) {
      tsBlockBuilder.getTimeColumnBuilder().writeLong(0L);
      tsBlockBuilder
          .getColumnBuilder(0)
          .writeBinary(new Binary(entry.getKey().getFullPath(), TSFileConfig.STRING_CHARSET));
      tsBlockBuilder.getColumnBuilder(1).writeLong(entry.getValue());
      tsBlockBuilder.declarePosition();
    }
    countMap.clear();
    return tsBlockBuilder.build();
  }
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