presto-main/src/main/java/io/prestosql/execution/scheduler/AllAtOnceExecutionSchedule.java/Visitor/visitUnion
class Visitor:        @Override
        public Void visitUnion(UnionNode node, Void context)
        {
            for (PlanNode subPlanNode : node.getSources()) {
                subPlanNode.accept(this, context);
            }

            return null;
        }