PostgreSQL 两个-table 约束无法正常工作

Question

我运行下面的代码：

-- Table describing messages
CREATE TABLE messages
(
 id serial PRIMARY KEY NOT NULL,
 text TEXT  -- Message can have or not have text
);

-- Table describing media attached to messages
CREATE TABLE messages_attachments
(
 message_id integer NOT NULL REFERENCES messages,
 -- Messages can have any number of attachments, including 0
 attachment_id TEXT NOT NULL
);

-- Messages must have either text or at least one attachment
CREATE FUNCTION message_has_text_or_attachments(integer) RETURNS bool STABLE
AS
$$
 SELECT
  EXISTS(SELECT 1 FROM messages_attachments WHERE message_id = )
 OR
  (SELECT text IS NOT NULL FROM messages WHERE id = );
$$ LANGUAGE SQL;

ALTER TABLE messages ADD CONSTRAINT nonempty_message CHECK ( message_has_text_or_attachments(id) );

-- Insert a message with no text and no attachments. Should fail, but it does not
INSERT INTO messages(text) VALUES (NULL);

SELECT *, message_has_text_or_attachments(id) FROM messages;

我预计它会在 INSERT 行失败，因为插入的行违反了检查约束（我们正在插入一条文本为 NULL 的消息，并且该消息没有附件） , 但它运行s 成功并且下一个查询 returns (1, NULL, false)（这里是一个稍微修改过的 example函数定义（由于数据库版本的原因，使用撇号而不是美元符号）。

一件更有趣的事情是，如果我在添加 CONSTRAINT 之前更改命令的顺序和 INSERT 行，那么 PostgreSQL 将无法 ALTER table, 因为“check constraint "nonempty_message" is violated by some row”.

为什么 PostgreSQL 允许插入违反约束的行？我在函数定义的某处弄错了吗？对于如何应用约束以及它们可以依赖哪些 table 是否有一些限制？是 PostgreSQL 错误吗？

Answer 1

来自the docs：

PostgreSQL does not support CHECK constraints that reference table data other than the new or updated row being checked. While a CHECK constraint that violates this rule may appear to work in simple tests, it cannot guarantee that the database will not reach a state in which the constraint condition is false (due to subsequent changes of the other row(s) involved).

PostgreSQL 两个-table 约束无法正常工作

PostgreSQL two-table constaint not working properly

postgresql

check-constraints