presto-main/src/main/java/io/prestosql/cost/CostCalculatorUsingExchanges.java/CostEstimator/costForSource
class CostEstimator:        private PlanCostEstimate costForSource(PlanNode node, LocalCostEstimate localCost)
        {
            verify(node.getSources().isEmpty(), "Unexpected sources for %s: %s", node, node.getSources());
            return new PlanCostEstimate(localCost.getCpuCost(), localCost.getMaxMemory(), localCost.getMaxMemory(), localCost.getNetworkCost(), localCost);
        }