@Override
  public long calculateMaxPeekMemory() {
    return DEFAULT_MAX_TSBLOCK_SIZE_IN_BYTES;
  }
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