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。對于一個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: preferencesCreate Table: CREATE TABLE `preferences` ( `numericId` int(10) unsigned NOT NULL, `receiveNotifications` tinyint(1) DEFAULT NULL, PRIMARY KEY (`numericId`)) ENGINE=InnoDB DEFAULT CHARSET=latin11 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 |
| ---TRANSACTION 4A18101, ACTIVE 12 sec2 lock struct(s), heap size 376, 1 row lock(s)MySQL thread id 3, OS thread handle 0x7ff2200cd700, query id 35 localhost msandboxTrx read view will not see trx with id >= 4A18102, sees < 4A18102TABLE LOCK table `test`.`preferences` trx id 4A18101 lock mode IXRECORD LOCKS space id 31766 page no 3 n bits 72 index `PRIMARY` of table `test`.`preferences` trx id 4A18101 lock_mode X |
這是為什么呢，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: 0mysql2> 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: 0mysql1> INSERT INTO preferences (numericId, receiveNotifications) VALUES ('1', '1'); -- This one goes into LOCK WAITmysql2> 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么的容易，因此一定要避免這種情況——如果來自于事務的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。
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