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MySQL你到底都加了什么锁？
2022-08-01 19:13:00 【wang0907】
写在前面
本文一起看下sql语句delete from t1 where id = 10在不同的场景下都会加哪些锁，但是在此之前必须先来明确几个概念，下面先来一起看下。
MVCC
MVCC全称是multi version concurrency control,即版本并发控制，其本质是一种协议，一种数据读操作的协议，规定读分为快照度和当前读，其中快照读基于数据快照读取，读取到的可能是数据的某历史版本，这在不同隔离级别中大有用武之地，另一种当前读，是需要读取最新的数据版本。在MySQL中InnoDB对当前协议提供了具体的实现，我们通过InnoDB来看下哪些操作是快照读，哪些操作又是当前读。
- 快照读
select ? from ? where ?;
- 当前读
select ? from ? where ? lock in share mode;
select ? from ? where ? for update;
insert into ? value (?,?);
update ? set ? where ?;
delete from ? where ?;
其中只有select ? from ? where ? lock in share mode;是Sshare共享锁，其他的都是Xexclusive排他锁。
两阶段锁协议
即在不同的阶段加锁和释放锁，以innodb事务为例，在语句执行前获取锁，在其所在的事务提交时释放锁。
隔离级别
RU：读未提交，读取到其他事务没有提交的修改，实际中不会使用。
RC：读以提交，读取到其他事物已经提交的修改。
RR：可重复读，每次读取到的数据的信息一样。
Serializale：串行，实际中几乎不会使用。
1：不同场景分析
准备数据：
drop table t;
CREATE TABLE `t` (
`id` int(11) NOT NULL,
`c` int(11) DEFAULT NULL,
`d` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `c` (`c`)
) ENGINE=InnoDB;
insert into t values(0,10,100),(5,50,500),
(100,1000,10000),(150,1500,15000),(200,2000,2000),(250,2500,25000);
1.1：主键+RC
注意将数据恢复到初始状态！！！
- 修改数据库隔离级别为RC
mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| REPEATABLE-READ |
+-----------------------+
1 row in set (0.00 sec)
mysql> select @@global.transaction_isolation;
ERROR 1193 (HY000): Unknown system variable 'transaction_isolation'
mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| REPEATABLE-READ |
+-----------------------+
1 row in set (0.00 sec)
mysql> set global transaction isolation level read committed;
Query OK, 0 rows affected (0.00 sec)
新开一个会话查看是否修改成功！！！
mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| READ-COMMITTED |
+-----------------------+
1 row in set (0.00 sec)
- 开启两个会话A，B
- 在会话A执行操作
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> delete from t where id=5;
Query OK, 1 row affected (0.00 sec)
- 在会话B执行操作
mysql> update t set c=c where id=5;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
可以看到阻塞等待，只有id=5的record排它行锁。注意锁是加在聚簇索引上。此时加锁如下(图仅示意)：
RC不存在间隙锁，所以需要考虑，其实这种场景就算是RR也不会有间隙锁，因为id=5的行是存在的，也是只会加record 的X锁。
1.2：唯一索引+RC
注意将数据恢复到初始状态！！！
- 修改数据库隔离级别为RC
mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| REPEATABLE-READ |
+-----------------------+
1 row in set (0.00 sec)
mysql> select @@global.transaction_isolation;
ERROR 1193 (HY000): Unknown system variable 'transaction_isolation'
mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| REPEATABLE-READ |
+-----------------------+
1 row in set (0.00 sec)
mysql> set global transaction isolation level read committed;
Query OK, 0 rows affected (0.00 sec)
新开一个会话查看是否修改成功！！！
mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| READ-COMMITTED |
+-----------------------+
1 row in set (0.00 sec)
- 添加唯一索引
mysql> alter table t add unique index uni_idx_d(`d`);
Query OK, 0 rows affected (0.10 sec)
Records: 0 Duplicates: 0 Warnings: 0
- 开启两个会话A，B
- 在会话A执行操作
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> delete from t where id=5;
Query OK, 1 row affected (0.00 sec)
- 在会话B执行操作
mysql> update t set c=c where id=5;
阻塞
此时show processlist结果如下：
mysql> show processlist;
+----+------+-----------+------+---------+------+----------+-----------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+------+---------+------+----------+-----------------------------+
| 7 | root | localhost | test | Sleep | 29 | | NULL |
| 9 | root | localhost | test | Query | 23 | updating | update t set c=c where id=5 |
| 11 | root | localhost | NULL | Query | 0 | starting | show processlist |
+----+------+-----------+------+---------+------+----------+-----------------------------+
3 rows in set (0.01 sec)
其中| 9 | root | localhost | test | Query | 23 | updating | update t set c=c where id=5 |就是阻塞的线程，从状态updating可以看出来是被行锁阻塞了，但是这里其实是有两把行X锁的，一把是二级索引树c上的，另一把是聚簇索引上的,这是因为同一记录上的更新/删除操作需要串行执行，不然就会出现数据一致性的问题。
1.3：非唯一索引+RC
注意将数据恢复到初始状态！！！
- 修改数据库隔离级别为RC
mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| REPEATABLE-READ |
+-----------------------+
1 row in set (0.00 sec)
mysql> select @@global.transaction_isolation;
ERROR 1193 (HY000): Unknown system variable 'transaction_isolation'
mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| REPEATABLE-READ |
+-----------------------+
1 row in set (0.00 sec)
mysql> set global transaction isolation level read committed;
Query OK, 0 rows affected (0.00 sec)
新开一个会话查看是否修改成功！！！
mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| READ-COMMITTED |
+-----------------------+
1 row in set (0.00 sec)
- 新开2个会话A，B
- 在会话A执行如下操作
mysql> start transaction with consistent snapshot;
Query OK, 0 rows affected, 1 warning (0.01 sec)
mysql> delete from t where c=10;
Query OK, 1 row affected (0.00 sec)
- 在会话B执行如下操作
mysql> delete from t where c=10;
阻塞
此时show processlist结果如下：
mysql> show processlist;
+----+------+-----------+------+---------+------+----------+-----------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+------+---------+------+----------+-----------------------------+
| 7 | root | localhost | test | Sleep | 29 | | NULL |
| 9 | root | localhost | test | Query | 23 | updating | update t set c=c where id=5 |
| 11 | root | localhost | NULL | Query | 0 | starting | show processlist |
+----+------+-----------+------+---------+------+----------+-----------------------------+
3 rows in set (0.01 sec)
其中| 9 | root | localhost | test | Query | 23 | updating | update t set c=c where id=5 |就是阻塞的线程，从状态updating可以看出来是被行锁阻塞了，但是这里其实是有c=10的行数 * 2把行X锁的，一半的锁是二级索引树c上的，另一半是聚簇索引上的,这是因为同一记录上的更新/删除操作需要串行执行，不然就会出现数据一致性的问题。
1.4：普通列+RC
注意将数据恢复到初始状态！！！
- 修改数据库隔离级别为RC
mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| REPEATABLE-READ |
+-----------------------+
1 row in set (0.00 sec)
mysql> select @@global.transaction_isolation;
ERROR 1193 (HY000): Unknown system variable 'transaction_isolation'
mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| REPEATABLE-READ |
+-----------------------+
1 row in set (0.00 sec)
mysql> set global transaction isolation level read committed;
Query OK, 0 rows affected (0.00 sec)
新开一个会话查看是否修改成功！！！
mysql> select @@global.tx_isolation;
+-----------------------+
| @@global.tx_isolation |
+-----------------------+
| READ-COMMITTED |
+-----------------------+
1 row in set (0.00 sec)
- 新开2个会话A，B
- 在会话A执行如下操作
mysql> delete from t where d=25000;
Query OK, 1 row affected (0.01 sec)
- 在会话B执行如下操作
mysql> update t set c=c where d=25000;
阻塞
show processlist结果如下：
mysql> show processlist;
+----+------+-----------+------+---------+------+----------+--------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------+------+---------+------+----------+--------------------------------+
| 7 | root | localhost | test | Sleep | 164 | | NULL |
| 9 | root | localhost | test | Query | 3 | updating | update t set c=c where d=25000 |
| 11 | root | localhost | NULL | Query | 0 | starting | show processlist |
+----+------+-----------+------+---------+------+----------+--------------------------------+
3 rows in set (0.00 sec)
其中| 9 | root | localhost | test | Query | 3 | updating | update t set c=c where d=25000 |就代表被record lock阻塞了，这里加锁的行是聚簇索引上所有满足d=25000的行。
1.5：主键列+RR
注意将数据恢复到初始状态！！！
- 修改数据库隔离级别为RR
Repeatable Read
mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set, 1 warning (0.01 sec)
mysql> set global transaction isolation level repeatable read;
Query OK, 0 rows affected (0.01 sec)
新开一个会话，查看RR隔离级别是否生效
mysql> show variables like 'tx_isolation';
+---------------+-----------------+
| Variable_name | Value |
+---------------+-----------------+
| tx_isolation | REPEATABLE-READ |
+---------------+-----------------+
1 row in set (0.05 sec)
- 开启两个会话A，B
- 在会话A执行操作
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> delete from t where id=5;
Query OK, 1 row affected (0.00 sec)
- 在会话B执行操作
mysql> update t set c=c where id=5;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
可以看到阻塞等待，只有id=5的record排它行锁。注意锁是加在聚簇索引上。此时加锁如下(图仅示意)：
1.6：唯一索引列+RR
同1.2：唯一索引+RC，主键索引树和二级索引树分别一般record 的写锁。
1.7：普通索引列+RR
注意将数据恢复到初始状态！！！
- 修改数据库隔离级别为RR
Repeatable Read
mysql> select @@tx_isolation;
+----------------+
| @@tx_isolation |
+----------------+
| READ-COMMITTED |
+----------------+
1 row in set, 1 warning (0.01 sec)
mysql> set global transaction isolation level repeatable read;
Query OK, 0 rows affected (0.01 sec)
新开一个会话，查看RR隔离级别是否生效
mysql> show variables like 'tx_isolation';
+---------------+-----------------+
| Variable_name | Value |
+---------------+-----------------+
| tx_isolation | REPEATABLE-READ |
+---------------+-----------------+
1 row in set (0.05 sec)
- 开启两个会话A，B
- 在会话A执行操作
mysql> start transaction with consistent snapshot;
Query OK, 0 rows affected (0.00 sec)
mysql> delete from t where c=1000;
Query OK, 1 row affected (0.00 sec)
- 在会话A执行操作
证明间隙锁的存在：
mysql> begin;
mysql> insert into t values(RAND() * 900 + 100, 6, RAND() * 900 + 100); /*间隙（负无穷,10）*/
Query OK, 1 row affected (0.00 sec)
mysql> insert into t values(RAND() * 900 + 100, 36, RAND() * 900 + 100); /*间隙（10,50）*/
Query OK, 1 row affected (0.00 sec)
mysql> insert into t values(RAND() * 900 + 100, 999, RAND() * 900 + 100); /*间隙（50,1000）*/
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into t values(RAND() * 900 + 100, 1333, RAND() * 900 + 100); /*间隙（1000,1500）*/
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into t values(RAND() * 900 + 100, 1833, RAND() * 900 + 100); /*间隙（1500,2000）*/
Query OK, 1 row affected (0.00 sec)
mysql> insert into t values(RAND() * 900 + 100, 2433, RAND() * 900 + 100); /*间隙（2000,2500）*/
Query OK, 1 row affected (0.00 sec)
mysql> insert into t values(RAND() * 900 + 100, 99999, RAND() * 900 + 100); /*间隙（2500,正无穷）*/
Query OK, 1 row affected (0.00 sec)
可以看到间隙（50,1000）和（1000,1500）被加上了锁。
证明行写锁的存在：
mysql> update t set c=c where id=100;
阻塞
show processlist结果：
mysql> show processlist;
+----+------+-----------------+------+---------+------+----------+-------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+------+-----------------+------+---------+------+----------+-------------------------------+
| 13 | root | localhost:39648 | test | Query | 2 | Updating | update t set c=c where id=100 |
| 14 | root | localhost:39948 | test | Sleep | 2271 | | NULL |
| 15 | root | localhost:44283 | NULL | Query | 0 | NULL | show processlist |
+----+------+-----------------+------+---------+------+----------+-------------------------------+
1.8：普通列+RR
- 启动2个会话A，B
- 在会话A启动事务，并使用当前读执行查询
mysql> start transaction;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from t where d=500 for update;
+----+------+------+
| id | c | d |
+----+------+------+
| 5 | 50 | 500 |
+----+------+------+
1 row in set (0.00 sec)
- 在会话B插入
ID=9的数据
可以看到阻塞等待，实际上加的间隙锁如下图：
其中区间的就是间隙锁，即影响的行的ID值落到区间内都会被间隙锁所阻塞，另外上面的数字的代表是行锁，即(-∞，0)间隙锁，0行锁，(0，5)间隙锁，5行锁，(5,100)间隙锁，100行锁，(100,150)间隙锁，150行锁，(150,200)间隙锁，200行锁，(200,250)间隙锁，250行锁，(250,+∞)间隙锁，其中间隙锁和行锁的组合我们叫做next-key lock,使用左开右闭的格式来表示，即(-∞，0]next-key lock，(0，5]next-key lock，(5,10]next-key lock，(10,15]next-key lock，(15,20]next-key lock，(20,25]next-key lock，(25,supremum]next-key lock,也就是只要是落到了这些区间的就都会被阻塞，为什么要在所有这些间隙都加锁呢，因为所有间隙都有可能产生满足where=500条件的新数据！！！其实此时就是任何值都会阻塞。
如下一些insert的操作（因为insert操作可能会破坏where d=500的条件，所以都会阻塞）：
mysql> insert into t values(9,9,9);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update t set d=5 where id=0;
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into t values (7,7,7);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into t values (23,23,23);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> insert into t values (9999,9999,9999);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
如下是一些update的操作，分为两种情况，第一种是update有匹配的行，此时就会被阻塞，因为可能会破坏where d=500的条件，而当没有匹配的行时，因为不可能会破坏where d=500的条件，所以不会被阻塞，如下分别测试有匹配行和没有匹配行的情况(是否有匹配行已经给出了注释，另外set部分不重要，重要的是where部分)：
mysql> update t set c=c where id=0; /*有匹配行*/
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update t set d=9090 where id=0; /*有匹配行*/
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update t set d=8 where id=0; /*有匹配行*/
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update t set c=c where id=0; /*有匹配行*/
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql> update t set d=8 where id=2; /*无匹配行*/
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0 Changed: 0 Warnings: 0
mysql> update t set d=8 where id=2222; /*无匹配行*/
Query OK, 0 rows affected (0.00 sec)
Rows matched: 0 Changed: 0 Warnings: 0
mysql> update t set d=8 where id=250; /*有匹配行*/
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql>
看到这里，不知道你有没有一个疑问，比如insert操作insert into t values (7,7,7)生成的新数据，并不满足where=500的条件，比如update操作update t set c=c where id=0; /*有匹配行*/也不会产生满足where=500的数据，但是为什么就阻塞了呢？MySQL就这么笨不会加上这个判断吗？我认为不这样做的原因是，代价太大！！！我们的例子肯定是很好判断的，但是实际的情况可就是千变万化了，比如insert into t values (7,7,select dVal from xxx where xxx in(selet ...)),判断的成本必定不可预估。
写在后面
参考文章列表：
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