@Override
  public PlanNode clone() {
    return new GroupNode(id, null, orderingScheme, partitionKeyCount);
  }
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