当两个事务相互等待以获得锁时，就会发生死锁。例子:

Tx 1:锁定A，然后锁定B Tx 2:锁定B，然后是A

关于死锁有许多问题和答案。每次插入/更新/或删除一行时，都会获得一个锁。为了避免死锁，必须确保并发事务不会按照可能导致死锁的顺序更新行。一般来说，即使在不同的事务中，也尽量以相同的顺序获取锁(例如，总是先获取表A，然后获取表B)。

Another reason for deadlock in database can be missing indexes. When a row is inserted/update/delete, the database needs to check the relational constraints, that is, make sure the relations are consistent. To do so, the database needs to check the foreign keys in the related tables. It might result in other lock being acquired than the row that is modified. Be sure then to always have index on the foreign keys (and of course primary keys), otherwise it could result in a table lock instead of a row lock. If table lock happen, the lock contention is higher and the likelihood of deadlock increases.

如果有人还在纠结这个问题:

我也遇到过类似的问题，2个请求同时到达服务器。没有像下面这样的情况:

T1:
    BEGIN TRANSACTION
    INSERT TABLE A
    INSERT TABLE B
    END TRANSACTION

T2:
    BEGIN TRANSACTION
    INSERT TABLE B
    INSERT TABLE A
    END TRANSACTION

所以，我很困惑为什么会发生死锁。

然后我发现由于外键的存在，两个表之间存在父子关系。当我在子表中插入一条记录时，事务正在父表的行上获取一个锁。在此之后，我试图更新父行，这触发了锁的提升到EXCLUSIVE一。由于第二个并发事务已经持有一个SHARED锁，因此导致死锁。

参考网址:https://blog.tekenlight.com/2019/02/21/database-deadlock-mysql.html

您可以尝试通过首先将要删除的每一行的键插入到临时表(如下面的伪代码)来操作该删除作业

create temporary table deletetemp (userid int);

insert into deletetemp (userid)
  select userid from onlineusers where datetime <= now - interval 900 second;

delete from onlineusers where userid in (select userid from deletetemp);

像这样分解它的效率较低，但它避免了在删除过程中持有key-range锁的需要。

另外，修改选择查询以添加where子句，排除时间超过900秒的行。这避免了对cron作业的依赖，并允许您重新安排它以减少运行频率。

Theory about the deadlocks: I don't have a lot of background in MySQL but here goes... The delete is going to hold a key-range lock for datetime, to prevent rows matching its where clause from being added in the middle of the transaction, and as it finds rows to delete it will attempt to acquire a lock on each page it is modifying. The insert is going to acquire a lock on the page it is inserting into, and then attempt to acquire the key lock. Normally the insert will wait patiently for that key lock to open up but this will deadlock if the delete tries to lock the same page the insert is using because thedelete needs that page lock and the insert needs that key lock. This doesn't seem right for inserts though, the delete and insert are using datetime ranges that don't overlap so maybe something else is going on.

http://dev.mysql.com/doc/refman/5.1/en/innodb-next-key-locking.html

@Omry Yadan的答案(https://stackoverflow.com/a/2423921/1810962)可以用ORDER by简化。

改变

DELETE FROM onlineusers 
WHERE datetime <= now() - INTERVAL 900 SECOND

to

DELETE FROM onlineusers 
WHERE datetime <= now() - INTERVAL 900 SECOND
ORDER BY ID

以保持您删除项目的顺序一致。此外，如果在单个事务中执行多个插入，请确保它们也始终按id排序。

根据mysql delete文档:

如果指定了ORDER BY子句，则按指定的顺序删除行。

你可以在这里找到参考资料:https://dev.mysql.com/doc/refman/8.0/en/delete.html

对于使用Spring的Java程序员，我使用一个AOP方面避免了这个问题，该方面可以自动重试陷入瞬时死锁的事务。

更多信息请参见@RetryTransaction Javadoc。

当两个事务相互等待以获得锁时，就会发生死锁。例子:

Tx 1:锁定A，然后锁定B Tx 2:锁定B，然后是A

关于死锁有许多问题和答案。每次插入/更新/或删除一行时，都会获得一个锁。为了避免死锁，必须确保并发事务不会按照可能导致死锁的顺序更新行。一般来说，即使在不同的事务中，也尽量以相同的顺序获取锁(例如，总是先获取表A，然后获取表B)。

Another reason for deadlock in database can be missing indexes. When a row is inserted/update/delete, the database needs to check the relational constraints, that is, make sure the relations are consistent. To do so, the database needs to check the foreign keys in the related tables. It might result in other lock being acquired than the row that is modified. Be sure then to always have index on the foreign keys (and of course primary keys), otherwise it could result in a table lock instead of a row lock. If table lock happen, the lock contention is higher and the likelihood of deadlock increases.