presto-main/src/main/java/io/prestosql/operator/DriverContext.java/DriverContext/getOperatorStats
class DriverContext:    public List<OperatorStats> getOperatorStats()
    {
        return operatorContexts.stream()
                .flatMap(operatorContext -> operatorContext
                        .getNestedOperatorStats()
                        .stream())
                .collect(toImmutableList());
    }