MySQL 事务的隔离级别及锁操作演示

WindWant

发布于 2022-05-10 09:26:28

5140

发布于 2022-05-10 09:26:28

文章被收录于专栏：后端码事后端码事

MySQL 版本：5.7

安装环境：MAC OS

一、测试数据

测试数据库：test；测试表：tt

CREATE TABLE `tt` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(100) DEFAULT NULL,
  KEY `name_idx` (`name`),
  KEY `id_idx` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

插入测试数据：

insert into tt value(1, "a”);
insert into tt value(1, "b”);
insert into tt value(2, “b");

二、数据库服务设置

1、事务隔离级别设置

mysql> set global transaction_isolation = 'repeatable-read';
Query OK, 0 rows affected (0.00 sec)
mysql> select @@transaction_isolation;
+-------------------------+
| @@transaction_isolation |
+-------------------------+
| READ-UNCOMMITTED        |
+-------------------------+

2、锁等待时间设置

mysql> set global innodb_lock_wait_timeout=5;
Query OK, 0 rows affected (0.00 sec)
 
mysql> select @@innodb_lock_wait_timeout;
+----------------------------+
| @@innodb_lock_wait_timeout |
+----------------------------+
|                          5 |
+----------------------------+

3、附注

更改设置后，后续开启的连接 Session 才会生效。

三、读未提交（READ-UNCOMMITTED）

开启两个连接 Session：

Session 1	Session 2
开启事务，更新 id 为 2 的记录 name 为 “ss" ，保持事务未提交： Query OK, 0 rows affected (0.00 sec) mysql> select * from tt; +------+------+ \| id \| name \| +------+------+ \| 1 \| a \| \| 1 \| b \| \| 2 \| b \| +------+------+ 3 rows in set (0.00 sec) mysql> update tt set name = 'ss' where id = 2; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0
	开启事务，查询 id 为 2 的记录 name 值： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where id = 2; +------+------+ \| id \| name \| +------+------+ \| 2 \| ss \| +------+———+ 事务 2 可以查询到事务 1 未提交的数据变更。对于事务 2 来说，这条数据是脏数据。

四、读已提交（READ-COMMITTED）

解决 READ-UNCOMMITTED 隔离级别下产生的脏读现象。

设置事务隔离级别：

mysql> set global transaction_isolation = 'read-committed';
Query OK, 0 rows affected (0.00 sec)

重新开启测试 Session，查询事务隔离级别：

mysql> select @@transaction_isolation;
+-------------------------+
| @@transaction_isolation |
+-------------------------+
| READ-COMMITTED          |
+-------------------------+

Session 1	Session 2
开启事务，更新 id 为 2 的记录 name 为 “ssr”： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> update tt set name = 'ssr' where id = 2; Query OK, 1 row affected (0.01 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from tt where id = 2; +------+------+ \| id \| name \| +------+------+ \| 2 \| ssr \| +------+------+ 1 row in set (0.01 sec)
	查询数据，无法查询到事务 1 未提交的数据： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where id = 2; +------+------+ \| id \| name \| +------+------+ \| 2 \| b \| +------+------+ 1 row in set (0.00 sec)
提交事务： mysql> commit ; Query OK, 0 rows affected (0.01 sec)
	查询数据，得到的是事务 1 中已提交的数据变更： mysql> select * from tt where id = 2; +------+------+ \| id \| name \| +------+------+ \| 2 \| ssr \| +------+------+ 1 row in set (0.00 sec) 对于事务 2 来说，在事务 1 提交前后，获取到的数据是不一样的，即不可重复读问题。

五、可重复读（REPEATABLE-READ）

解决 READ-COMMITTED 隔离级别下产生的不可重复读现象。

Session 1中设置事务隔离级别：

mysql> set global transaction_isolation = 'repeatable-read';
Query OK, 0 rows affected (0.01 sec)

重新开启事务，查询隔离级别：

mysql> select @@transaction_isolation;
+-------------------------+
| @@transaction_isolation |
+-------------------------+
| REPEATABLE-READ         |
+-------------------------+
1 row in set (0.00 sec)

Session 1	Session 2
	Session 2 开启事务，查询数据： mysql> begin; Query OK, 0 rows affected (0.00 sec) Database changed mysql> select * from tt where id = 2; +------+------+ \| id \| name \| +------+------+ \| 2 \| b \| +------+------+ 1 row in set (0.00 sec)
更新 id 为 2 的记录 name 为 “ssrr”，并提交事务： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> update tt set name = 'ssrr' where id = 2; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> commit; Query OK, 0 rows affected (0.01 sec) mysql> select * from tt where id = 2; +------+------+ \| id \| name \| +------+------+ \| 2 \| ssrr \| +------+------+ 1 row in set (0.00 sec)
	Session 2 重新查询数据： mysql> select * from tt where id = 2; +------+------+ \| id \| name \| +------+------+ \| 2 \| b \| +------+------+ 当前数据未变。但是问题是，事务 1 已经进行了数据变更，并且提交，事务 2 无法获取所查记录最新变更信息。

为什么事务 2 前后两次相同查询所得的数据是一样的？

一致性读（consistent read）查询模式：基于【某一时刻】的【数据快照】提供读查询结果。无论查询的数据是否被其它事务所改变。这个【某一时刻】在 repeatable-read 隔离级别下为事务中第一次执行查询操作的时间点，read-committed 隔离级别下，数据快照会在每一次执行一致性读操作时进行重置。

幻读

如何避免：加X锁

Next-key lock：Record lock + Gap lock

六、关于 Next-key lock 加锁

调整表 tt 索引及数据：

mysql> show create table tt;
+-------+-------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                      |
+-------+-------------------------------------------------------+
| tt    | CREATE TABLE `tt` (
  `id` int(11) NOT NULL,
  `name` varchar(100) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_age` (`age`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+-------------------------------------------------------+
1 row in set (0.00 sec)
mysql> select * from tt;
+-----+------+------+
| id  | name | age  |
+-----+------+------+
|  90 | aa   |   10 |
| 102 | bb   |   15 |
| 108 | cc   |   20 |
| 130 | dd   |   25 |
| 150 | ee   |   30 |
+-----+------+------+

1、等值条件

对于使用唯一性索引：加的锁为 Record lock

Session 1	Session 2
开启事务，查询 id 为 108 记录加 X lock： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where id = 108 for update; +-----+------+------+ \| id \| name \| age \| +-----+------+------+ \| 108 \| cc \| 20 \| +-----+------+------+ 1 row in set (0.01 sec)
	开启事务，记录前后紧邻 gap 插入记录： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into tt value(106, 'bc', 16); Query OK, 1 row affected (0.00 sec) mysql> insert into tt value(110, 'cd', 22); Query OK, 1 row affected (0.00 sec) 记录均可成功插入

对于使用非唯一性索引：加的锁为 Record lock + Gap lock 前后紧邻 gap

：首先加锁 (15, 20]，因为是非唯一索引，继续向后查找到第一个不满足条件的元素 25 加 gap lock (20, 25)

Session 1	Session 2
开启事务，查询 age 为 20 记录加 X lock： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where age = 20 for update; +-----+------+------+ \| id \| name \| age \| +-----+------+------+ \| 108 \| cc \| 20 \| +-----+------+------+ 1 row in set (0.00 sec)
	开启事务，记录紧邻前后 gap 插入记录： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into tt value(106, 'bc', 18); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(110, 'cd', 22); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 操作均被 block。紧邻 gap 以外插入记录： mysql> insert into tt value(100, 'ab', 12); Query OK, 1 row affected (0.00 sec) mysql> insert into tt value(140, 'de', 27); Query OK, 1 row affected (0.00 sec) 记录均可成功插入

Session 1

Session 2

开启事务，查询 age 为 20 记录加 X lock： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where age = 20 for update; +-----+------+------+ | id | name | age | +-----+------+------+ | 108 | cc | 20 | +-----+------+------+ 1 row in set (0.00 sec)

开启事务，记录紧邻前后 gap 插入记录： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into tt value(106, 'bc', 18); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(110, 'cd', 22); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 操作均被 block。紧邻 gap 以外插入记录： mysql> insert into tt value(100, 'ab', 12); Query OK, 1 row affected (0.00 sec) mysql> insert into tt value(140, 'de', 27); Query OK, 1 row affected (0.00 sec) 记录均可成功插入

对于不使用索引的：加锁为全部记录及gap

Session1	Session2
开启事务，查询 name 为 ‘cc’ 记录加 X lock： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where name = 'cc' for update; +-----+------+------+ \| id \| name \| age \| +-----+------+------+ \| 108 \| cc \| 20 \| +-----+------+------+
	开启事务，各个间隙尝试插入记录： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into tt value(80, 'pa', 5); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(95, 'ab', 13); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(105, 'bc', 18); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(120, 'cd', 23); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(140, 'de', 28); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(160, 'en', 35); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 操作均被 block。更新记录： mysql> update tt set age = 21 where name = 'cc'; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> update tt set age = 16 where name = 'bb'; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 操作均被 block。

Session1

Session2

开启事务，查询 name 为 ‘cc’ 记录加 X lock： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where name = 'cc' for update; +-----+------+------+ | id | name | age | +-----+------+------+ | 108 | cc | 20 | +-----+------+------+

开启事务，各个间隙尝试插入记录： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into tt value(80, 'pa', 5); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(95, 'ab', 13); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(105, 'bc', 18); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(120, 'cd', 23); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(140, 'de', 28); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(160, 'en', 35); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 操作均被 block。更新记录： mysql> update tt set age = 21 where name = 'cc'; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> update tt set age = 16 where name = 'bb'; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 操作均被 block。

2、范围条件

使用唯一索引：

Session 1	Session 2
开启事务，查询 id 为 108 的记录用以更新 mysql> select * from tt where id >= 108 and id < 109 for update; +-----+------+------+ \| id \| name \| age \| +-----+------+------+ \| 108 \| cc \| 20 \| +-----+------+------+ 1 row in set (0.01 sec)
	开启事务，间隙插入记录： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into tt value(120, 'cd', 23); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> update tt set age = 26 where id = 130; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 操作被 block。加锁说明： >= 108 条件：主键索引加锁为 Record lock 记录 108 < 109 条件：因为不存在 109 记录，所以继续向右遍历至 130 不满足，加锁 (108, 130]
重新开启事务，右侧条件改为开区间 130： mysql> rollback; Query OK, 0 rows affected (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where id >= 108 and id < 130 for update; +-----+------+------+ \| id \| name \| age \| +-----+------+------+ \| 108 \| cc \| 20 \| +-----+------+------+ 1 row in set (0.01 sec)
	开启事务，操作 130 后间隙插入记录及更新 id 为 130 记录： mysql> insert into tt value(135, 'ce', 32); Query OK, 1 row affected (0.01 sec) mysql> rollback; Query OK, 0 rows affected (0.01 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> update tt set age = 26 where id = 130; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 插入操作成功，更新操作被 block。加锁说明： < 130 条件：存在 130 记录，加锁 (108, 130]
重新开启事务，右侧条件改为闭区间 130 mysql> rollback; Query OK, 0 rows affected (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where id >= 108 and id <= 130 for update; +-----+------+------+ \| id \| name \| age \| +-----+------+------+ \| 108 \| cc \| 20 \| \| 130 \| dd \| 25 \| +-----+------+------+
	开启事务，操作 130 后间隙插入记录及更新 id 为 150 记录： mysql> rollback; Query OK, 0 rows affected (0.01 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into tt value(140, 'de', 28); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> update tt set age = 35 where id = 150; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 插入操作成功，更新操作被 block。加锁说明： <= 130 条件：存在 130 记录，加锁 (108, 130]，继续向右查询到不满足条件记录 150，加锁 (130, 150]

2、使用非唯一索引

Session 1	Session 2
开启事务，查询 age 范围记录用以更新： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where age >= 20 and age < 21 for update; +-----+------+------+ \| id \| name \| age \| +-----+------+------+ \| 108 \| cc \| 20 \| +-----+------+------+
	开启事务，间隙插入记录： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into tt value(105, 'bc', 18); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(120, 'cd', 23); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> insert into tt value(140, 'de', 28); Query OK, 1 row affected (0.00 sec) mysql> update tt set name = 'test' where age = 25; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 加锁说明： >= 20 条件：非唯一索引，加锁为 (15, 20] < 21 条件：因为不存在 21 记录，所以继续向右遍历至 25 不满足，加锁 (20, 25]
重新开启事务，右侧条件改为开区间 25 mysql> rollback; Query OK, 0 rows affected (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where age >= 20 and age < 25 for update; +-----+------+------+ \| id \| name \| age \| +-----+------+------+ \| 108 \| cc \| 20 \| +-----+------+------+
	开启事务，操作 25 后间隙插入记录及更新 id 为 25 记录： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into tt value(140, 'de', 28); Query OK, 1 row affected (0.00 sec) mysql> update tt set name = 'test' where age = 25; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 插入操作成功，更新操作被 block。加锁说明： < 25 条件：存在 25 记录，加锁 (20, 25]
重新开启事务，右侧条件改为闭区间 25 mysql> rollback; Query OK, 0 rows affected (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from tt where age >= 20 and age <= 25 for update; +-----+------+------+ \| id \| name \| age \| +-----+------+------+ \| 108 \| cc \| 20 \| \| 130 \| dd \| 25 \| +-----+------+------+
	开启事务，操作 25 后间隙插入记录及更新 age 为 30 记录： mysql> rollback; Query OK, 0 rows affected (0.00 sec) mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> insert into tt value(140, 'de', 28); ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction mysql> update tt set name = 'test' where age = 30; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction 插入操作成功，更新操作被 block。加锁说明： <= 25 条件：存在 25 记录，加锁 (20, 25]，继续向右查询到不满足条件记录 30，加锁 (25, 30]

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Session 1	Session 2
	Session 2 开启事务，查询数据： mysql> begin; Query OK, 0 rows affected (0.00 sec) Database changed mysql> select * from tt where id = 2; +------+------+ \| id \| name \| +------+------+ \| 2 \| b \| +------+------+ 1 row in set (0.00 sec)
更新 id 为 2 的记录 name 为 “ssrr”，并提交事务： mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> update tt set name = 'ssrr' where id = 2; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> commit; Query OK, 0 rows affected (0.01 sec) mysql> select * from tt where id = 2; +------+------+ \| id \| name \| +------+------+ \| 2 \| ssrr \| +------+------+ 1 row in set (0.00 sec)
	Session 2 重新查询数据： mysql> select * from tt where id = 2; +------+------+ \| id \| name \| +------+------+ \| 2 \| b \| +------+------+ 当前数据未变。但是问题是，事务 1 已经进行了数据变更，并且提交，事务 2 无法获取所查记录最新变更信息。