presto-main/src/main/java/io/prestosql/sql/planner/ExpressionExtractor.java/ExpressionExtractor/visitPlan
class ExpressionExtractor:        @Override
        protected Void visitPlan(PlanNode node, Void context)
        {
            if (recursive) {
                return super.visitPlan(node, context);
            }
            return null;
        }