default PlanNode resolve(PlanNode node) {
    if (node instanceof GroupReference) {
      return resolveGroup(node).collect(toOptional()).get();
    }
    return node;
  }
Resolves a node by materializing GroupReference nodes representing symbolic references to other
nodes. This method is deprecated since is assumes group contains only one node.

<p>If the node is not a GroupReference, it returns the argument as is.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/planner/iterative/Lookup.java