@Override
  public PlanNode clone() {
    return new CollectNode(id, outputSymbols);
  }
CollectNode output the content of children. Normally it will output the child one by one, but in
some cases, while some children are blocked, it may output the content of other children.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/planner/node/CollectNode.java