presto-main/src/main/java/io/prestosql/cost/CostCalculatorUsingExchanges.java/CostEstimator/visitUnion
class CostEstimator:        @Override
        public PlanCostEstimate visitUnion(UnionNode node, Void context)
        {
            // Cost will be accounted either in CostCalculatorUsingExchanges#CostEstimator#visitExchange
            // or in CostCalculatorWithEstimatedExchanges#CostEstimator#visitUnion
            // This stub is needed just to avoid the cumulative cost being set to unknown
            return costForStreaming(node, LocalCostEstimate.zero());
        }