public void serialize(DataOutputStream stream) throws IOException {
      ReadWriteIOUtils.write(groupingKeys.size(), stream);
      for (Symbol symbol : groupingKeys) {
        Symbol.serialize(symbol, stream);
      }
      ReadWriteIOUtils.write(groupingSetCount, stream);
      ReadWriteIOUtils.write(globalGroupingSets.size(), stream);
      for (int globalGroupingSet : globalGroupingSets) {
        ReadWriteIOUtils.write(globalGroupingSet, stream);
      }
    }
@return whether this node should produce default output in case of no input pages. For example
    for query:
    <p>SELECT count(*) FROM nation WHERE nationkey < 0
    <p>A default output of "0" is expected to be produced by FINAL aggregation operator.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/planner/node/AggregationNode.java