presto-main/src/main/java/io/prestosql/cost/CostCalculatorUsingExchanges.java/CostCalculatorUsingExchanges/getSourcesEstimations
class CostCalculatorUsingExchanges:        private Stream<PlanCostEstimate> getSourcesEstimations(PlanNode node)
        {
            return node.getSources().stream()
                    .map(sourcesCosts::getCost);
        }