@Override
  protected void serializeAttributes(DataOutputStream stream) throws IOException {
    PlanNodeType.TABLE_GROUP_NODE.serialize(stream);
    orderingScheme.serialize(stream);
    ReadWriteIOUtils.write(partitionKeyCount, stream);
  }
orderingScheme may include two parts: PartitionKey and OrderKey. It marks the number of
PartitionKey.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/planner/node/GroupNode.java