在PostgreSQL中使用SELECT FOR UPDATE选择进入 [英] SELECT INTO with SELECT FOR UPDATE in PostgreSQL

问题描述

假设我有一个message关系，在其中保存了使用以下命令创建的消息:

Assume I have a message relation where I save messages that was created with this command:

CREATE TABLE message 
(
     id serial primary key, 
     foo1 integer, 
     foo2 integer, 
     foo3 text
)

我们有一个获取消息并将其从关系中删除的函数，如下所示:

And we have a function that gets a message and deletes it from the relation, like this:

CREATE FUNCTION get_and_delete_message(p_foo1 integer)
RETURNS TABLE(r_id integer, r_foo1 integer, r_foo2 integer, r_foo3 text) AS $$
DECLARE
  message_id integer;
BEGIN
    SELECT id INTO message_id FROM message WHERE foo1 = p_foo1 LIMIT 1;
    RETURN QUERY SELECT * FROM message WHERE foo1 = p_foo1 LIMIT 1;
    DELETE FROM message where id = message_id;
END;
$$ LANGUAGE plpgsql;

假设 READ_COMMITTED 隔离级别，则可能是来自两个用户的两个并发事务返回相同的消息，尽管显然只有一个会删除/获取它.这不是我的应用程序所希望的行为，我希望仅由一个用户读取一条消息.

Assuming READ_COMMITTED isolation level it could be that two concurrent transactions from two users return the same message, although obviously only ones deletes/gets it. This is not the desired behavior for my application, I want one message to be read by only one user.

假设 REPEATABLE_READ 不会发生.

Assuming REPEATABLE_READ this wouldn't happen.

但是在阅读了 我认为也许仍然可以使用READ_COMMITTED级别并按以下方式更改get_and_delete_message函数:

But after reading about FOR UPDATE I thought perhaps it's still possible to use READ_COMMITTED level and change the get_and_delete_message function as following:

   ...
    BEGIN
        SELECT id INTO message_id FROM message WHERE foo1 = p_foo1 LIMIT 1;
        RETURN QUERY SELECT * FROM message WHERE foo1 = p_foo1 LIMIT 1 FOR UPDATE;
        DELETE FROM message where id = message_id;
    END;
    ...

据我了解，在第二个SELECT中使用FOR UPDATE实际上将锁定返回的行，直到事务结束，因此，如果我们有2个并发事务，则实际上只有一个将返回并删除该消息.

From my understanding, using FOR UPDATE in the second SELECT will actually lock the returned rows until the end of the transaction, so if we have 2 concurrent transactions only one will actually return and delete the message.

是这种情况吗?还是我也应该做SELECT id INTO message_id FROM message WHERE foo1 = p_foo1 LIMIT 1 FOR UPDATE?我找不到将 SELECT INTO 与FOR UPDATE.有什么想法吗?

Is this the case? Or should I also do SELECT id INTO message_id FROM message WHERE foo1 = p_foo1 LIMIT 1 FOR UPDATE? I couldn't find any information on combining SELECT INTO with FOR UPDATE. Any ideas on that?

推荐答案

您可以在单个语句中完成此操作，无需选择:

You can do that in a single statement, no need for a select:

CREATE FUNCTION get_and_delete_message(p_foo1 integer)
RETURNS TABLE(r_id integer, r_foo1 integer, r_foo2 integer, r_foo3 text) 
AS $$
  DELETE FROM message 
    where foo1 = p_foo1 
  returning *;
END;
$$ LANGUAGE sql;

这将删除行，然后作为delete语句的结果返回所有已删除的行.

This will delete the rows and then return all deleted rows as a result of the delete statement.

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