MySQL 死鎖套路：三個事務插入有一個回滾

這個是很是經典的一個場景，在網上搜索插入意向鎖形成的死鎖，絕大部分都會指向這個例子

表結構

CREATE TABLE `t1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `a` varchar(5),
  `b` varchar(5),
  PRIMARY KEY (`id`),
  UNIQUE KEY `uk_name` (`a`,`b`)
);
複製代碼

三個事務的 insert 語句都是insert ignore into t1(a, b)values("1", "1");

復現步驟以下：

t1	t2	t3	備註
begin	begin	begin
insert			成功
	insert		把 t1 的隱式鎖提高爲 X 鎖，t2 進入進入 S 鎖等待
		insert	t3 進入進入 S 鎖等待
rollback;			t1 回滾之後，釋放 X 鎖，t2 和 t3 同時拿到了 S 鎖
	ok	deadlock	t2 和 t3 都想拿插入意向鎖 X 鎖，形成死鎖條件

死鎖日誌以下

------------------------
LATEST DETECTED DEADLOCK
------------------------
181101 23:22:59
*** (1) TRANSACTION:
TRANSACTION 5032, ACTIVE 11 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 4 lock struct(s), heap size 1248, 2 row lock(s), undo log entries 1
MySQL thread id 5, OS thread handle 0x70000d736000, query id 125 localhost root update
insert ignore into t1(a, b)values("1", "1")
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 56 page no 4 n bits 584 index `uk_name` of table `d1`.`t1` trx id 5032 lock_mode X locks gap before rec insert intention waiting
Record lock, heap no 139 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
 0: len 3; hex 313031; asc 101;;
 1: len 3; hex 313031; asc 101;;
 2: len 4; hex 800007b1; asc     ;;

*** (2) TRANSACTION:
TRANSACTION 5033, ACTIVE 6 sec inserting
mysql tables in use 1, locked 1
4 lock struct(s), heap size 1248, 2 row lock(s), undo log entries 1
MySQL thread id 6, OS thread handle 0x70000d779000, query id 126 localhost root update
insert ignore into t1(a, b)values("1", "1")
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 56 page no 4 n bits 584 index `uk_name` of table `d1`.`t1` trx id 5033 lock mode S locks gap before rec
Record lock, heap no 139 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
 0: len 3; hex 313031; asc 101;;
 1: len 3; hex 313031; asc 101;;
 2: len 4; hex 800007b1; asc     ;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 56 page no 4 n bits 584 index `uk_name` of table `d1`.`t1` trx id 5033 lock_mode X locks gap before rec insert intention waiting
Record lock, heap no 139 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
 0: len 3; hex 313031; asc 101;;
 1: len 3; hex 313031; asc 101;;
 2: len 4; hex 800007b1; asc     ;;

*** WE ROLL BACK TRANSACTION (2)
複製代碼

這裏咱們第一次提到了插入意向鎖（insert intention lock）

插入意向鎖（insert intention lock） 對已有數據行的修改與刪除，必須增強互斥鎖 X 鎖，那對於數據的插入，是否還須要加這麼強的鎖，來實施互斥呢？插入意向鎖，孕育而生。 插入意向鎖是間隙鎖（Gap Locks）的一種，它是專門針對 insert 操做的，也是爲數很少的在 RC 級別下產生 Gap 鎖狀況