Question

我很想知道为什么当主键不存在时，使用主键的两个并发DELETE后跟INSERT语句会导致MySQL出现死锁。这个例子是用最简单的形式来说明问题的。

这是设置。

> SELECT @@GLOBAL.tx_isolation, @@tx_isolation;
+-------------------------+------------------+
| @@GLOBAL.tx_isolation   | @@tx_isolation   |
|-------------------------+------------------|
| REPEATABLE-READ         | REPEATABLE-READ  |
+-------------------------+------------------+
1 row in set
Time: 0.002s

> select version();
+-------------+
| version()   |
|-------------|
| 5.7.12      |
+-------------+
1 row in set
Time: 0.002s

create table lock_test ( id int(11) not null, primary key (`id`) );

下面，1>代表一个mysql终端，2>代表另一个终端。

1> begin;
1> delete from lock_test where id = 1;

2> begin;
2> delete from lock_test where id = 2;

1> insert into lock_test values (1); -- hangs

2> insert into lock_test values (2);
*** deadlock ***

这是show engine innodb status输出：

------------------------
LATEST DETECTED DEADLOCK
------------------------
2018-06-06 16:15:18 0x70000ba52000
*** (1) TRANSACTION:
TRANSACTION 807765, ACTIVE 46 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 1136, 2 row lock(s)
MySQL thread id 620, OS thread handle 123145496289280, query id 43097 localhost ::1 root update
insert into lock_test values (1)
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 6819 page no 3 n bits 72 index PRIMARY of table `content_graph`.`lock_test` trx id 807765 lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

*** (2) TRANSACTION:
TRANSACTION 807766, ACTIVE 37 sec inserting
mysql tables in use 1, locked 1
3 lock struct(s), heap size 1136, 2 row lock(s)
MySQL thread id 617, OS thread handle 123145497681920, query id 43099 localhost ::1 root update
insert into lock_test values (2)
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 6819 page no 3 n bits 72 index PRIMARY of table `content_graph`.`lock_test` trx id 807766 lock_mode X
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 6819 page no 3 n bits 72 index PRIMARY of table `content_graph`.`lock_test` trx id 807766 lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
 0: len 8; hex 73757072656d756d; asc supremum;;

*** WE ROLL BACK TRANSACTION (2)

请注意，如果您首先使用ID 1和2插入记录，然后重复上面的序列，则不存在死锁。

我的感觉是，因为密钥不在索引中（两者都在追加），删除必须锁定更多的索引（主要登陆的页面更多）但我想确保我有理解正确。

Answer 1

＆＃34;差距＆＃34;因为预期有人可能会尝试插入我要删除的行而被锁定。

或者，以另一种方式来看......完全处理每一个奇怪的情况都太慢了。因此，InnoDB选择有效地处理大多数情况，并在罕见的奇怪案例上进行投资。

底线：和它一起生活。您将陷入僵局。你不一定能够理解它们。但是您的代码需要恢复 - 只需回滚并返回BEGIN。

为什么两个并发的delete + insert语句在空表上死锁？

1 个答案: