presto-main/src/main/java/io/prestosql/cost/CostCalculatorUsingExchanges.java/CostEstimator/visitEnforceSingleRow
class CostEstimator:        @Override
        public PlanCostEstimate visitEnforceSingleRow(EnforceSingleRowNode node, Void context)
        {
            return costForAccumulation(node, LocalCostEstimate.zero());
        }