@Override
  public PlanNode replaceChildren(List<PlanNode> newChildren) {
    return new GroupNode(
        id, Iterables.getOnlyElement(newChildren), 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