presto-main/src/main/java/io/prestosql/sql/planner/optimizations/PlanNodeDecorrelator.java/DecorrelatingVisitor/visitEnforceSingleRow
class DecorrelatingVisitor:        @Override
        public Optional<DecorrelationResult> visitEnforceSingleRow(EnforceSingleRowNode node, Void context)
        {
            Optional<DecorrelationResult> childDecorrelationResultOptional = node.getSource().accept(this, null);
            return childDecorrelationResultOptional.filter(result -> result.atMostSingleRow);
        }