@Override
  public String toString() {
    return "GroupNode-" + this.getPlanNodeId();
  }
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