这篇文章将为大家详细讲解有关MySQL为什么慎用if not exists写法，小编觉得挺实用的，因此分享给大家做个参考，希望大家阅读完这篇文章后可以有所收获。
版本 5.6.14
环境
CREATE TABLE `t1` (
`id` int(11) NOT NULL,
`batchid` int(11) DEFAULT NULL,
`state` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
INSERT INTO `t1` (`id`,`batchid`,`state`) VALUES (1,1,1);
INSERT INTO `t1` (`id`,`batchid`,`state`) VALUES (2,1,1);
drop procedure if exists p;
delimiter $$
CREATE PROCEDURE `p`()
begin
if not exists (select * from t1 where id=1) then
select 'id=1 not exists';
end if;
end $$
delimiter ;
按照如下时间轴执行 ,窗口二居然被阻塞了.
查看 show engine innodb status;
发现如下信息:
---TRANSACTION 15252974, ACTIVE 2 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 376, 1 row lock(s)
MySQL thread id 38, OS thread handle 0x1d88, query id 4663 localhost 127.0.0.1 root statistics
select * from t1 where id=1
LIMIT 0, 1000
for update
------- TRX HAS BEEN WAITING 2 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 2344 page no 3 n bits 72 index `PRIMARY` of table `mvbox`.`t1` trx id 15252974 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 000000e8bdc4; asc ;;
2: len 7; hex 030003c0100437; asc 7;;
3: len 4; hex 80000001; asc ;;
4: len 4; hex 80000001; asc ;;
------------------
---TRANSACTION 15252973, ACTIVE 15 sec
2 lock struct(s), heap size 376, 1 row lock(s)
MySQL thread id 36, OS thread handle 0x35c, query id 4615 localhost 127.0.0.1 root cleaning up
过程中 if not exists 的查询居然上了锁..
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