从包含相互指向的 FK 的表中删除记录

Question

背景：这里有点意外的 DBA。我有一个类似 reddit 的网站，用户可以在其中提交指向各种互联网内容的链接，然后可以在每个帖子下发表评论。这个应用程序——我们称之为链接——有两个相应的表来存储数据：link和publicreply（即每个链接的评论关联）。

问题：由于相互依赖的 FK 约束，我似乎无法从这两个表中删除记录（用于维护）。需要指导来解决这种情况。

详细信息：每个Publicreply对象都存储Link与其关联的对象的 FK 。此外，每个Link对象还保存对与其相关联的最新公开回复的引用。这会造成所有Publicreply对象都有LinkFK 的情况，反之亦然。如：

Table "public.links_link"
        Column        |           Type           |                        Modifiers                        
----------------------+--------------------------+---------------------------------------------------------
 id                   | integer                  | not null default nextval('links_link_id_seq'::regclass)
 description          | text                     | not null
 submitter_id         | integer                  | not null
 submitted_on         | timestamp with time zone | not null
 url                  | character varying(250)   | not null
 image_file           | character varying(100)   | 
 reply_count          | integer                  | default 0
 latest_reply_id      | integer                  | 
 is_visible           | boolean                  | default true
Indexes:
    "links_link_pkey" PRIMARY KEY, btree (id)
    "links_link_submitter_id" btree (submitter_id)
Foreign-key constraints:
    "links_link_submitter_id_fkey" FOREIGN KEY (submitter_id) REFERENCES auth_user(id) DEFERRABLE INITIALLY DEFERRED
    "publicreplyposter_link_fkey" FOREIGN KEY (latest_reply_id) REFERENCES links_publicreply(id) ON UPDATE CASCADE ON DELETE CASCADE
Referenced by:
    TABLE "links_publicreply" CONSTRAINT "links_publicreply_answer_to_id_fkey" FOREIGN KEY (answer_to_id) REFERENCES links_link(id) DEFERRABLE INITIALLY DEFERRED

---

 Table "public.links_publicreply"
     Column      |           Type           |                           Modifiers                            
-----------------+--------------------------+----------------------------------------------------------------
 id              | integer                  | not null default nextval('links_publicreply_id_seq'::regclass)
 submitted_by_id | integer                  | not null
 answer_to_id    | integer                  | not null
 submitted_on    | timestamp with time zone | not null
 description     | text                     | not null
Indexes:
    "links_publicreply_pkey" PRIMARY KEY, btree (id)
    "links_publicreply_answer_to_id" btree (answer_to_id)
    "links_publicreply_submitted_by_id" btree (submitted_by_id)
Foreign-key constraints:
    "links_publicreply_answer_to_id_fkey" FOREIGN KEY (answer_to_id) REFERENCES links_link(id) DEFERRABLE INITIALLY DEFERRED
    "links_publicreply_submitted_by_id_fkey" FOREIGN KEY (submitted_by_id) REFERENCES auth_user(id) DEFERRABLE INITIALLY DEFERRED
Referenced by:
    TABLE "links_link" CONSTRAINT "publicreplyposter_link_fkey" FOREIGN KEY (latest_reply_id) REFERENCES links_publicreply(id) ON UPDATE CASCADE ON DELETE CASCADE

Link在这种情况下如何从表中删除记录？而从Publicreply？我使用的是 postgresql 9.3.10。

Answer 1

该问题可以通过多种方式解决：

1. 首先我们注意到 FK 列之一可以为空。这允许从两个表中删除，在单个事务中使用三个语句并且不需要延迟约束。首先更新latest_reply_id为 null，然后删除 from PublicReply，然后 from Link：

   -- Delete one (or more) `Link` rows 
   -- and all the `Publicreply` rows associated with them:
   BEGIN ;
       UPDATE public.links_link
       SET latest_reply_id = NULL
       WHERE id IN (?, ?, ..., ?) ;              -- Link ids to be deleted
   
       DELETE FROM public.links_publicreply
       WHERE answer_to_id IN (?, ?, ..., ?) ;    -- Link ids to be deleted
   
       DELETE FROM public.links_link
       WHERE id IN (?, ?, ..., ?) ;              -- Link ids to be deleted
   COMMIT ;
   

2. 然后我们注意到其中一个外键约束已经被定义为可延迟的。这允许从两个表中删除，在单个事务中使用两个语句：

   -- Delete one (or more) `Link` rows 
   -- and all the `Publicreply` rows associated with them:
   BEGIN ;
       DELETE FROM public.links_link
       WHERE id IN (?, ?, ..., ?) ;              -- Link ids to be deleted
   
       DELETE FROM public.links_publicreply
       WHERE answer_to_id IN (?, ?, ..., ?) ;    -- Link ids to be deleted
   COMMIT ;
   

3. 使用可修改的 CTE，我们可以在单个语句中从两个表中删除。不需要为此推迟约束。例子：

   -- Delete one (or more) `Link` rows 
   -- and all the `Publicreply` rows associated with them:
   WITH del_link AS
     ( DELETE FROM public.links_link
       WHERE id IN (?, ?, ..., ?)                -- Link ids to be deleted
       RETURNING id
     )
   DELETE FROM public.links_publicreply
   WHERE answer_to_id IN (TABLE del_link) ;
   

删除 fromPublicReply会稍微复杂一些，具体取决于要求，但可以使用上述任何一种方法来完成。有什么要求？

• 删除 a PublicReply，它的父级Link和所有相关的回复？

• 删除一个PublicReply，如果是最新的回复，将父级更改Link为指向上一个回复？如果它是唯一的，将其设置为NULL？

• 删除一个PublicReply，如果是最新的回复，将父级更改Link为指向上一个回复？如果它是唯一的，也删除父级Link？