我要评论在mysql数据库中，间隙锁（gap locks）和临键锁（next-key locks）是实现行级锁的一种方式，它们主要用于支持事务的隔离级别，尤其是在可重复读（repeatable read）和串行化（serializable）隔离级别下。间隙锁和临键锁共同确保了事务的隔离性和一致性，防止了幻读（phantom reads）的发生。
间隙锁（gap locks）
间隙锁锁定的是一个范围，但不包括记录本身。例如，如果事务a在范围(10, 20)上加了间隙锁，那么其他事务就不能在这个范围内插入新的记录。
临键锁（next-key locks）
临键锁是间隙锁和记录锁的结合，锁定一个范围并包括该范围内的最高记录。在mysql的innodb存储引擎中，默认情况下，所有行级锁都是临键锁。这意味着如果一个事务对某个范围内的记录加了临键锁，那么其他事务既不能在这个范围内插入新记录，也不能修改或删除这个范围内的任何现有记录。
版本：mysql-8.4.9
表结构：
create table `employees2` ( `id` int not null auto_increment, `name` varchar(24) not null default '' comment '姓名', `age` int not null default '0' comment '年龄', `position` varchar(20) not null default '' comment '职位', `hire_time` timestamp not null default current_timestamp comment '入职时间', primary key (`id`), key `age_index` (`age`) ) engine=innodb auto_increment=200104 default charset=utf8mb3 comment='员工记录表';
表数据：
mysql> select * from employees2; +----+------+-----+----------+---------------------+ | id | name | age | position | hire_time | +----+------+-----+----------+---------------------+ | 5 | a | 5 | dev | 2026-08-07 08:57:09 | | 10 | b | 10 | test | 2026-08-07 08:57:19 | | 15 | c | 15 | prod | 2026-08-07 08:57:38 | | 20 | d | 20 | prod2 | 2026-08-07 08:57:52 | +----+------+-----+----------+---------------------+ 4 rows in set (0.00 sec)
索引：age_index 为普通索引 ， rr 级别；
事务1： 开启事务，查询age>10 and age < 15 并加上排他锁。此时事务1会锁住10到15 区间。包括10，不包括15。[10,15)
begin; select * from employees2 where age >10 and age <15 for update;
事务2： age = 9 时，直接提交成功，无需等待事务1完成
mysql> begin; query ok, 0 rows affected (0.00 sec) mysql> insert into employees2 (id,name,age,position) values (21,'a',9,'aa'); query ok, 1 row affected (0.00 sec) mysql> commit; query ok, 0 rows affected (0.00 sec)
事务2： age = 10 ，必须等待事务1 提交后，才能提交。
mysql> begin; query ok, 0 rows affected (0.00 sec) mysql> insert into employees2 (id,name,age,position) values (22,'a',10,'aa'); error 1205 (hy000): lock wait timeout exceeded; try restarting transaction mysql>
执行结果：
正常执行:
t2 执行 insert into employees2 (id,name,age,position) values (22,‘a’,10,‘aa’); 后，t1 commit 提交后，t2获取到锁后正常提交。
结果：此范围查询间隙锁范围包括10；
事务1继续：
mysql> begin; query ok, 0 rows affected (0.00 sec) mysql> select * from employees2 where age > 10 and age < 15 for update; empty set (0.00 sec) mysql>
事务2： age = 15 是正常添加，age=15 不在事务1间隙锁范围
mysql> begin;
query ok, 0 rows affected (0.00 sec)
mysql> insert into employees2 (id,name,age,position) values (23,'a',15,'aa');
query ok, 1 row affected (0.00 sec)
mysql> commit
-> ;
query ok, 0 rows affected (0.00 sec)结果
事务2 继续： 插入age = 14 时，在事务1间隙锁范围内，无法插入。需要等待事务1 执行结束。
mysql> begin; query ok, 0 rows affected (0.00 sec) mysql> insert into employees2 (id,name,age,position) values (25,'a',14,'aa'); error 1205 (hy000): lock wait timeout exceeded; try restarting transaction
结果：此范围查询间隙锁范围不包括15 ；
所以：
select * from employees2 where age > 10 and age < 15 for update;
age普通索引，间隙锁范围是：包括10 ，但是不包括15 ，区间为 [10,15)
场景2 : select * from employees2 where age >= 10 and age <= 15 for update;
数据恢复
mysql> select * from employees2; +----+------+-----+----------+---------------------+ | id | name | age | position | hire_time | +----+------+-----+----------+---------------------+ | 5 | a | 5 | dev | 2026-08-07 08:57:09 | | 10 | b | 10 | test | 2026-08-07 08:57:19 | | 15 | c | 15 | prod | 2026-08-07 08:57:38 | | 20 | d | 20 | prod2 | 2026-08-07 08:57:52 | +----+------+-----+----------+---------------------+ 4 rows in set (0.00 sec)
事务1：
mysql> begin; query ok, 0 rows affected (0.00 sec) mysql> select * from employees2 where age >= 10 and age <= 15 for update; +----+------+-----+----------+---------------------+ | id | name | age | position | hire_time | +----+------+-----+----------+---------------------+ | 10 | b | 10 | test | 2026-08-07 08:57:19 | | 15 | c | 15 | prod | 2026-08-07 08:57:38 | +----+------+-----+----------+---------------------+ 2 rows in set (0.00 sec) mysql>
事务2： 插入 age = 4 没有问题，插入age=5 时，无法插入。原因事务1 的间隙锁 包含 age=5
mysql> begin; query ok, 0 rows affected (0.00 sec) mysql> insert into employees2 (id,name,age,position) values (21,'a',4,'aa'); query ok, 1 row affected (0.00 sec) mysql> insert into employees2 (id,name,age,position) values (22,'a',5,'aa'); error 1205 (hy000): lock wait timeout exceeded; try restarting transaction mysql>
正常情况：事务1 commit后，事务2立即插入age=5成功。
事务2继续： 事务2插入age=20 正常插入，插入age=19 时，无法插入。原因事务1 的间隙锁 包含 age=19**
mysql> begin; query ok, 0 rows affected (0.00 sec) mysql> insert into employees2 (id,name,age,position) values (23,'a',20,'aa'); query ok, 1 row affected (0.00 sec) mysql> insert into employees2 (id,name,age,position) values (24,'a',19,'aa'); error 1205 (hy000): lock wait timeout exceeded; try restarting transaction mysql>
age >= 10 and age <= 15 for update;
携带=时，普通索引 的区间 age 在 [5,20) ，包含5，不包含20.
场景3：select * from employees2 where age between 10 and 15 for update;;
与 age >= 10 and age <= 15 for update; 一致， 事务1间隙锁区间在 [5,20)
使用主键索引：
sql：select * from employees2 where id >= 10 and id <= 15 for update;
区间范围为：[10,15]
事务1：
mysql> begin; query ok, 0 rows affected (0.00 sec) mysql> select * from employees2 where id >= 10 and id <= 15 for update; +----+------+-----+----------+---------------------+ | id | name | age | position | hire_time | +----+------+-----+----------+---------------------+ | 10 | b | 10 | test | 2026-08-07 08:57:19 | | 15 | c | 15 | prod | 2026-08-07 08:57:38 | +----+------+-----+----------+---------------------+ 2 rows in set (0.00 sec) mysql>
事务2：
mysql> begin; query ok, 0 rows affected (0.00 sec) mysql> insert into employees2 (id,name,age,position) values (9,'a',5,'aa'); query ok, 1 row affected (0.00 sec) mysql> insert into employees2 (id,name,age,position) values (16,'a',5,'aa'); query ok, 1 row affected (0.00 sec) mysql> update employees2 set name = 'bb' where id = 10; error 1205 (hy000): lock wait timeout exceeded; try restarting transaction mysql> update employees2 set name = 'bb' where id = 15; error 1205 (hy000): lock wait timeout exceeded; try restarting transaction mysql>
sql: select * from employees2 where id > 10 and id < 15 for update;
事务1间隙锁区间范围为：(10,15)
事务1：
mysql> begin; query ok, 0 rows affected (0.00 sec) mysql> select * from employees2 where id > 10 and id < 15 for update; empty set (0.00 sec) mysql>
事务2：
mysql> begin; query ok, 0 rows affected (0.00 sec) mysql> mysql> update employees2 set name = 'bb' where id = 10; query ok, 1 row affected (0.00 sec) rows matched: 1 changed: 1 warnings: 0 mysql> update employees2 set name = 'bb' where id = 15; query ok, 1 row affected (0.00 sec) rows matched: 1 changed: 1 warnings: 0 mysql> insert into employees2 (id,name,age,position) values (11,'a',1,'aa'); error 1205 (hy000): lock wait timeout exceeded; try restarting transaction mysql> insert into employees2 (id,name,age,position) values (14,'a',1,'aa'); error 1205 (hy000): lock wait timeout exceeded; try restarting transaction mysql>
sql:select * from employees2 where id between 10 and 15 for update;
事务1 间隙锁区间范围为：[10,15]
事务1：
mysql> begin; query ok, 0 rows affected (0.00 sec) mysql> select * from employees2 where id between 10 and 15 for update; +----+------+-----+----------+---------------------+ | id | name | age | position | hire_time | +----+------+-----+----------+---------------------+ | 10 | b | 10 | test | 2026-08-07 08:57:19 | | 15 | c | 15 | prod | 2026-08-07 08:57:38 | +----+------+-----+----------+---------------------+ 2 rows in set (0.00 sec) mysql>
事务2：
mysql> begin
-> ;
query ok, 0 rows affected (0.00 sec)
mysql> insert into employees2 (id,name,age,position) values (9,'a',5,'aa');
query ok, 1 row affected (0.00 sec)
mysql> insert into employees2 (id,name,age,position) values (16,'a',5,'aa');
query ok, 1 row affected (0.00 sec)
mysql> update employees2 set name = 'bb' where id = 10;
error 1205 (hy000): lock wait timeout exceeded; try restarting transaction
mysql> update employees2 set name = 'bb' where id = 15;
error 1205 (hy000): lock wait timeout exceeded; try restarting transaction
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