在為一個客戶排除死鎖問題時我遇到了一個有趣的包括InnoDB間隙鎖的情形。對於一個WHERE子句不匹配任何行的非插入的寫操作中,我預期事務應該不會有鎖,但我錯了。讓我們看一下這張表及示例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: 0InnoDB狀態顯示這個UPDATE在主索引記錄上持有了一個X鎖:
---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
這是為什麼呢,Heikki在其bug報告中做了解釋,這很有意義,我知道修復起來很困難,但略帶厭惡地我又希望它能被差異化處理。為完成這篇文章,讓我證明下上面說到的死鎖情況,下面中mysql1是第一個會話,mysql2是另一個,查詢的順序如下:
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
現在你看到導致死鎖是多麼的容易,因此一定要避免這種情況——如果來自於事務的INSERT部分導致非插入的寫操作可能不匹配任何行的話,不要這樣做,使用REPLACE INTO或使用READ-COMMITTED事務隔離。