MySQL 死鎖套路：再來看一例走不一樣索引更新的例子

前面幾篇文章介紹了利用調試MySQL源碼的方式來調試鎖相關的信息，這裏利用這個工具來解決一個比較簡單的問題，線上的表字段較多，這裏簡單成爲了一個表

CREATE TABLE `t3` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `a` varchar(5) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '',
  `b` varchar(5) COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '',
  PRIMARY KEY (`id`),UNIQUE KEY `uk_a` (`a`), KEY `idx_b` (`b`) 
)
複製代碼

sql語句以下

t1
update t3 set b = '' where a = "1";

t2
update t3 set b = '' where b = "2";
複製代碼

咱們先用以前debug的方式來看一下，這裏兩條語句分別加了哪些鎖

第一條語句（經過惟一索引去更新記錄）

update t3 set b = '' where a = "1";

整理一下，加了3個X鎖，順序分別是

序號	索引	鎖類型
1	uk_a	X
2	PRIMARY	X
3	idx_b	X

第二條語句

update t3 set b = '' where b = "2";

整理一下，加了3個X鎖，順序分別是

序號	索引	鎖類型
1	idx_b	X
2	PRIMARY	X
3	idx_b	X

兩條語句從加鎖順序看起來就已經有構成死鎖的條件了

手動是比較難模擬的，寫個代碼去跑一下

立刻就出現了

------------------------
LATEST DETECTED DEADLOCK
------------------------
181102 12:45:05
*** (1) TRANSACTION:
TRANSACTION 50AF, ACTIVE 0 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 376, 2 row lock(s)
MySQL thread id 34, OS thread handle 0x70000d842000, query id 549 localhost 127.0.0.1 root Searching rows for update
update t3 set b = '' where b = "2"
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 67 page no 3 n bits 72 index `PRIMARY` of table `d1`.`t3` trx id 50AF lock_mode X locks rec but not gap waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 4; hex 80000001; asc     ;;
 1: len 6; hex 0000000050ae; asc     P ;;
 2: len 7; hex 03000001341003; asc     4  ;;
 3: len 1; hex 31; asc 1;;
 4: len 0; hex ; asc ;;

*** (2) TRANSACTION:
TRANSACTION 50AE, ACTIVE 0 sec updating or deleting
mysql tables in use 1, locked 1
4 lock struct(s), heap size 1248, 3 row lock(s), undo log entries 1
MySQL thread id 35, OS thread handle 0x70000d885000, query id 548 localhost 127.0.0.1 root Updating
update t3 set b = '' where a = "1"
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 67 page no 3 n bits 72 index `PRIMARY` of table `d1`.`t3` trx id 50AE lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 5; compact format; info bits 0
 0: len 4; hex 80000001; asc     ;;
 1: len 6; hex 0000000050ae; asc     P ;;
 2: len 7; hex 03000001341003; asc     4  ;;
 3: len 1; hex 31; asc 1;;
 4: len 0; hex ; asc ;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 67 page no 5 n bits 72 index `idx_b` of table `d1`.`t3` trx id 50AE lock_mode X locks rec but not gap waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
 0: len 1; hex 32; asc 2;;
 1: len 4; hex 80000001; asc     ;;

*** WE ROLL BACK TRANSACTION (1)
複製代碼

跟咱們線上的死鎖日誌如出一轍