presto-main/src/main/java/io/prestosql/cost/CostCalculatorUsingExchanges.java/CostEstimator/visitPlan
class CostEstimator:        @Override
        protected PlanCostEstimate visitPlan(PlanNode node, Void context)
        {
            // TODO implement cost estimates for all plan nodes
            return PlanCostEstimate.unknown();
        }