一个需要避免的 InnoDB 间隙锁

jopen 11年前

在为一个客户排除死锁问题时我遇到了一个有趣的包括InnoDB间隙锁的情形。对于一个WHERE子句不匹配任何行的非插入的写操作中,我预期事务应该不会有锁,但我错了。让我们看一下这张表及示例UPDATE。

mysql> SHOW CREATE TABLE preferences \G  *************************** 1. row ***************************         Table: preferences  Create Table: CREATE TABLE `preferences` (    `numericId` int(10) unsigned NOT NULL,    `receiveNotifications` tinyint(1) DEFAULT NULL,    PRIMARY KEY (`numericId`)  ) ENGINE=InnoDB DEFAULT CHARSET=latin1  1 row in set (0.00 sec)  mysql> BEGIN;  Query OK, 0 rows affected (0.00 sec)  mysql> SELECT COUNT(*) FROM preferences;  +----------+  | COUNT(*) |  +----------+  |        0 |  +----------+  1 row in set (0.01 sec)  mysql> UPDATE preferences SET receiveNotifications='1' WHERE numericId = '2';  Query OK, 0 rows affected (0.01 sec)  Rows matched: 0  Changed: 0  Warnings: 0

InnoDB状态显示这个UPDATE在主索引记录上持有了一个X锁:

---TRANSACTION 4A18101, ACTIVE 12 sec  2 lock struct(s), heap size 376, 1 row lock(s)  MySQL thread id 3, OS thread handle 0x7ff2200cd700, query id 35 localhost msandbox  Trx read view will not see trx with id >= 4A18102, sees < 4A18102  TABLE LOCK table `test`.`preferences` trx id 4A18101 lock mode IX  RECORD LOCKS space id 31766 page no 3 n bits 72 index `PRIMARY` of table `test`.`preferences` trx id 4A18101 lock_mode X

这是为什么呢,Heikki在其bug报告中做了解释,这很有意义,我知道修复起来很困难,但略带厌恶地我又希望它能被差异化处理。为完成这篇文章,让我证明下上面说到的死锁情况,下面中mysql1是第一个会话,mysql2是另一个,查询的顺序如下:

mysql1> BEGIN;  Query OK, 0 rows affected (0.00 sec)  mysql1> UPDATE preferences SET receiveNotifications='1' WHERE numericId = '1';  Query OK, 0 rows affected (0.00 sec)  Rows matched: 0 Changed: 0 Warnings: 0  mysql2> BEGIN;  Query OK, 0 rows affected (0.00 sec)  mysql2> UPDATE preferences SET receiveNotifications='1' WHERE numericId = '2';  Query OK, 0 rows affected (0.00 sec)  Rows matched: 0 Changed: 0 Warnings: 0  mysql1> INSERT INTO preferences (numericId, receiveNotifications) VALUES ('1', '1'); -- This one goes into LOCK WAIT  mysql2> INSERT INTO preferences (numericId, receiveNotifications) VALUES ('2', '1');  ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction

现在你看到导致死锁是多么的容易,因此一定要避免这种情况——如果来自于事务的INSERT部分导致非插入的写操作可能不匹配任何行的话,不要这样做,使用REPLACE INTO或使用READ-COMMITTED事务隔离。