presto-main/src/main/java/io/prestosql/sql/planner/DistributedExecutionPlanner.java/DistributedExecutionPlanner/visitEnforceSingleRow
class DistributedExecutionPlanner:        @Override
        public Map<PlanNodeId, SplitSource> visitEnforceSingleRow(EnforceSingleRowNode node, Void context)
        {
            return node.getSource().accept(this, context);
        }