presto-main/src/main/java/io/prestosql/cost/CostCalculatorWithEstimatedExchanges.java/ExchangeCostEstimator/visitUnion
class ExchangeCostEstimator:        @Override
        public LocalCostEstimate visitUnion(UnionNode node, Void context)
        {
            // this assumes that all union inputs will be gathered over the network
            // that is not aways true
            // but this estimate is better that returning UNKNOWN, as it sets
            // cumulative cost to unknown
            double inputSizeInBytes = getStats(node).getOutputSizeInBytes(node.getOutputSymbols(), types);
            return calculateRemoteGatherCost(inputSizeInBytes);
        }