Postgresql 强制列的唯一双向组合

Question

我正在尝试创建一个表，该表将在两个方向上强制执行相同类型的两列的唯一组合。例如。这是非法的:

col1 col2
   1    2
   2    1

我想出了这个，但它不起作用:

database=> \d+ friend;
                                Table "public.friend"
    Column    |           Type           | Modifiers | Storage  | Stats target | Description
--------------+--------------------------+-----------+----------+--------------+-------------
 user_id_from | text                     | not null  | extended |              |
 user_id_to   | text                     | not null  | extended |              |
 status       | text                     | not null  | extended |              |
 sent         | timestamp with time zone | not null  | plain    |              |
 updated      | timestamp with time zone |           | plain    |              |
Indexes:
    "friend_pkey" PRIMARY KEY, btree (user_id_from, user_id_to)
    "friend_user_id_to_user_id_from_key" UNIQUE CONSTRAINT, btree (user_id_to, user_id_from)
Foreign-key constraints:
    "friend_status_fkey" FOREIGN KEY (status) REFERENCES friend_status(name)
    "friend_user_id_from_fkey" FOREIGN KEY (user_id_from) REFERENCES user_account(login)
    "friend_user_id_to_fkey" FOREIGN KEY (user_id_to) REFERENCES user_account(login)
Has OIDs: no

是否可以在没有触发器或任何高级魔法的情况下仅使用约束来编写此代码？

Answer 1

不需要扩展的 Neil 解决方案的变体是:

create table friendz (
  from_id int,
  to_id int
);

create unique index ifriendz on friendz(greatest(from_id,to_id), least(from_id,to_id));

不过，Neil 的解决方案允许您使用任意数量的列。

我们都依赖于使用表达式来构建记录在案的索引 https://www.postgresql.org/docs/current/indexes-expressional.html