@Override
  public void close() throws Exception {
    for (Operator child : children) {
      if (child != null) {
        child.close();
      }
    }
    tsBlockBuilder = null;
  }
prepare next batch

@return true if need to break current loop, otherwise false
@throws Exception errors happened while fetching next batch data
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/process/last/LastQueryMergeOperator.java