postgressql 的 returning 关键字实现逻辑是怎样？

官网的 Synopsis：

[ WITH [ RECURSIVE ] with_query [, ...] ] DELETE FROM [ ONLY ] table_name [ * ] [ [ AS ] alias ] [ USING from_item [, ...] ] [ WHERE condition | WHERE CURRENT OF cursor_name ] [ RETURNING * | output_expression [ [ AS ] output_name ] [, ...] ]

returning 子句可以返回删除数据结果集。 那么它的底层逻辑是怎么样实现的呢，主要是 delete rows 和 table reader 的过程先后不太能搞清楚。