public PlanNode plan() {
      return plan;
    }
@param plan The optimized plan
@param changedPlanNodes The set of PlanNodeIds that were changed during optimization, as well
    as the new PlanNodeIds that were added to the optimized plan.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/planner/optimizations/AdaptivePlanOptimizer.java