presto-main/src/main/java/io/prestosql/sql/planner/LocalExecutionPlanner.java/LocalExecutionPlanner/visitEnforceSingleRow
class LocalExecutionPlanner:        @Override
        public PhysicalOperation visitEnforceSingleRow(EnforceSingleRowNode node, LocalExecutionPlanContext context)
        {
            PhysicalOperation source = node.getSource().accept(this, context);

            OperatorFactory operatorFactory = new EnforceSingleRowOperator.EnforceSingleRowOperatorFactory(context.getNextOperatorId(), node.getId());
            return new PhysicalOperation(operatorFactory, makeLayout(node), context, source);
        }