避免使用 MySQL 的 If Not Exists 语法的原因是什么

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.

（编辑：银川站长网）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!