presto-main/src/main/java/io/prestosql/sql/planner/PlanFragmenter.java/GroupedExecutionTagger/visitPlan
class GroupedExecutionTagger:        @Override
        protected GroupedExecutionProperties visitPlan(PlanNode node, Void context)
        {
            if (node.getSources().isEmpty()) {
                return GroupedExecutionProperties.notCapable();
            }
            return processChildren(node);
        }