mysql insert导致死锁的案例介绍
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两个insert语句发生死锁的案例。
一. 准备数据
CREATE TABLE `t1` (`a` int(11) NOT NULL,PRIMARY KEY (`a`)) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;insert into t1 values(1);mysql> insert into t1 values(1);Query OK, 1 row affected (0.20 sec)mysql> select * from t1;+---+| a |+---+| 1 |+---+1 row in set (0.00 sec)
二. 发起如下事务
会话1 | 会话2 | 会话3 |
begin; delete from t1 where a=1; | ||
begin; insert into t1 select 1; | ||
begin; insert into t1 select 1; | ||
Commit | ||
ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction |
简单理解下，删除数据的会话持有X锁，导致两条insert语句需要等待，这没问题，
但是为什么删除提交后，两个竞争关系出现了死锁。
show engine innodb status部分结果：
------------------------LATEST DETECTED DEADLOCK------------------------2020-05-17 13:50:24 0x7f660c3f0700*** (1) TRANSACTION:TRANSACTION 4377, ACTIVE 13 sec insertingmysql tables in use 1, locked 1LOCK WAIT 3 lock struct(s), heap size 1136, 2 row lock(s)MySQL thread id 3, OS thread handle 140076399294208, query id 59 localhost root executinginsert into t1 select 1*** (1) WAITING FOR THIS LOCK TO BE GRANTED:RECORD LOCKS space id 99 page no 3 n bits 72 index PRIMARY of table `ming`.`t1` trx id 4377 lock_mode X locks rec but not gap waitingRecord lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 6; hex 000000001114; asc ;; 2: len 7; hex 3400000144129f; asc 4 D ;;*** (2) TRANSACTION:TRANSACTION 4378, ACTIVE 10 sec inserting, thread declared inside InnoDB 1mysql tables in use 1, locked 13 lock struct(s), heap size 1136, 2 row lock(s)MySQL thread id 4, OS thread handle 140076268848896, query id 61 localhost root executinginsert into t1 select 1*** (2) HOLDS THE LOCK(S):RECORD LOCKS space id 99 page no 3 n bits 72 index PRIMARY of table `ming`.`t1` trx id 4378 lock mode SRecord lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 6; hex 000000001114; asc ;; 2: len 7; hex 3400000144129f; asc 4 D ;;*** (2) WAITING FOR THIS LOCK TO BE GRANTED:RECORD LOCKS space id 99 page no 3 n bits 72 index PRIMARY of table `ming`.`t1` trx id 4378 lock_mode X locks rec but not gap waitingRecord lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 32 0: len 4; hex 80000001; asc ;; 1: len 6; hex 000000001114; asc ;; 2: len 7; hex 3400000144129f; asc 4 D ;;*** WE ROLL BACK TRANSACTION (2)
可以看到会话2正在等待一个X行锁，会话3也在等待X行锁，但是同时持有一个S锁。
这个S锁是怎么来的呢？
当进行唯一性冲突检测时，需要先加一个 S 锁。
那么整个过程就是如下所示
会话1 | 会话2 | 会话3 |
begin; delete from t1 where a=1; 持有a=1就的X行锁 | ||
begin; insert into t1 select 1; 为了判断唯一性，请求a=1的next-key lock S锁被阻塞，等待 | ||
begin; insert into t1 select 1; 为了判断唯一性，请求a=1的next-key lock S锁被阻塞，等待 | ||
Commit 释放a=1上的锁 | ||
拿到a=1的next-key lock S锁，继续尝试拿a=1的X 行锁，但是被会话3的S锁阻塞 | 拿到a=1的next-key lock S锁，继续尝试拿a=1的X 行锁，尝试拿到a=1的X 行锁，但是被会话2的S锁阻塞。 触发死锁 ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction |
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