presto-main/src/main/java/io/prestosql/operator/EnforceSingleRowOperator.java/EnforceSingleRowOperator/createOperator
class EnforceSingleRowOperator:        @Override
        public Operator createOperator(DriverContext driverContext)
        {
            checkState(!closed, "Factory is already closed");
            OperatorContext operatorContext = driverContext.addOperatorContext(operatorId, planNodeId, EnforceSingleRowOperator.class.getSimpleName());
            return new EnforceSingleRowOperator(operatorContext);
        }