准备表结构
DROP TABLE IF EXISTS `t_student`; CREATE TABLE `t_student` (`id` int NOT NULL,`no` char(5) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NOT NULL,`name` varchar(64) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NOT NULL,`age` int NOT NULL,`score` int NOT NULL,PRIMARY KEY (`id`) USING BTREE ) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci ROW_FORMAT = Dynamic;-- ---------------------------- -- Records of t_student -- ---------------------------- INSERT INTO `t_student` VALUES (15, 'S0001', 'Bob', 25, 34); INSERT INTO `t_student` VALUES (18, 'S0002', 'Alice', 24, 77); INSERT INTO `t_student` VALUES (20, 'S0003', 'Jim', 24, 5); INSERT INTO `t_student` VALUES (30, 'S0004', 'Eric', 23, 91); INSERT INTO `t_student` VALUES (37, 'S0005', 'Tom', 22, 22); INSERT INTO `t_student` VALUES (49, 'S0006', 'Tom', 25, 83); INSERT INTO `t_student` VALUES (50, 'S0007', 'Rose', 23, 89);
一、 Insert语句分析
1. 开启事物，执行语句
mysql> begin; Query OK, 0 rows affected (0.00 sec)mysql> insert into t_student values(56,'S0008','Dany',23,89); Query OK, 1 row affected (0.00 sec)
2. 查看锁信息
mysql> select * from data_locks\G; *************************** 1. row ***************************ENGINE: INNODBENGINE_LOCK_ID: 2564937557728:1065:2564927920152 ENGINE_TRANSACTION_ID: 16710THREAD_ID: 52EVENT_ID: 29OBJECT_SCHEMA: testOBJECT_NAME: t_studentPARTITION_NAME: NULLSUBPARTITION_NAME: NULLINDEX_NAME: NULL OBJECT_INSTANCE_BEGIN: 2564927920152LOCK_TYPE: TABLELOCK_MODE: IXLOCK_STATUS: GRANTEDLOCK_DATA: NULL 1 row in set (0.00 sec)
说明：插入语句只是 在表上加入了意向排他锁(IX)，意向锁不会阻止其他事务进行插入操作，注意id=56这条记录此时没有被提交，是对其他事务不可见的（RR级别下）此时当前事务隐式持有id=56的记录锁，但如果其他事务执行update t_student set score=100 where id>20;也会被阻塞，因为该事务要获取id>20的所有Next Key Lock，获取id=56的Next Key Lock是获取不到的，需要等待。
一、 Update语句分析
mysql> begin; Query OK, 0 rows affected (0.00 sec)mysql> update t_student set score = 100 where id = 25; Query OK, 0 rows affected (0.00 sec) Rows matched: 0 Changed: 0 Warnings: 0
查看锁信息
mysql> select * from data_locks\G; *************************** 1. row ***************************ENGINE: INNODBENGINE_LOCK_ID: 2564937557728:1065:2564927920152 ENGINE_TRANSACTION_ID: 16711THREAD_ID: 52EVENT_ID: 33OBJECT_SCHEMA: testOBJECT_NAME: t_studentPARTITION_NAME: NULLSUBPARTITION_NAME: NULLINDEX_NAME: NULL OBJECT_INSTANCE_BEGIN: 2564927920152LOCK_TYPE: TABLELOCK_MODE: IXLOCK_STATUS: GRANTEDLOCK_DATA: NULL *************************** 2. row ***************************ENGINE: INNODBENGINE_LOCK_ID: 2564937557728:3:4:5:2564926668824 ENGINE_TRANSACTION_ID: 16711THREAD_ID: 52EVENT_ID: 33OBJECT_SCHEMA: testOBJECT_NAME: t_studentPARTITION_NAME: NULLSUBPARTITION_NAME: NULLINDEX_NAME: PRIMARY OBJECT_INSTANCE_BEGIN: 2564926668824LOCK_TYPE: RECORDLOCK_MODE: X,GAPLOCK_STATUS: GRANTEDLOCK_DATA: 30 2 rows in set (0.00 sec)
共加了两个锁，分别是：
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表锁：IX 类型的意向锁；
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行锁：X 类型的间隙锁；
这里我们重点关注行锁，图中 LOCK_TYPE 中的 RECORD 表示行级锁，而不是记录锁的意思，通过 LOCK_MODE 可以确认是 next-key 锁，还是间隙锁，还是记录锁：-
如果 LOCK_MODE 为 X，说明是 next-key 锁；
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如果 LOCK_MODE 为 X, REC_NOT_GAP，说明是记录锁；
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如果 LOCK_MODE 为 X, GAP，说明是间隙锁；因此，此时事务 A 在主键索引（INDEX_NAME : PRIMARY）上加的是间隙锁，锁范围是(20, 30)。
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说明 在表上加入了IX锁，在记录上加入了X锁和GAP锁，注意GAP锁和GAP锁并不互斥。GAP锁与插入意向锁互斥。
mysql> begin; Query OK, 0 rows affected (0.00 sec)mysql> update t_student set score = 100 where id = 20; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0
查看锁信息
mysql> select * from data_locks\G; *************************** 1. row ***************************ENGINE: INNODBENGINE_LOCK_ID: 2564937557728:1065:2564927920152 ENGINE_TRANSACTION_ID: 16716THREAD_ID: 52EVENT_ID: 37OBJECT_SCHEMA: testOBJECT_NAME: t_studentPARTITION_NAME: NULLSUBPARTITION_NAME: NULLINDEX_NAME: NULL OBJECT_INSTANCE_BEGIN: 2564927920152LOCK_TYPE: TABLELOCK_MODE: IXLOCK_STATUS: GRANTEDLOCK_DATA: NULL *************************** 2. row ***************************ENGINE: INNODBENGINE_LOCK_ID: 2564937557728:3:4:4:2564926668824 ENGINE_TRANSACTION_ID: 16716THREAD_ID: 52EVENT_ID: 37OBJECT_SCHEMA: testOBJECT_NAME: t_studentPARTITION_NAME: NULLSUBPARTITION_NAME: NULLINDEX_NAME: PRIMARY OBJECT_INSTANCE_BEGIN: 2564926668824LOCK_TYPE: RECORDLOCK_MODE: X,REC_NOT_GAPLOCK_STATUS: GRANTEDLOCK_DATA: 20 2 rows in set (0.00 sec)
说明 在表上加入了IX锁，在记录上加入了X锁，此时没有加GAP锁。
update t_student set score = 100 where id = 25; 表中并不存在id=25的记录。但是这里锁住了（20-30）这个区间，其他事务此时向这个区间插入数据会被阻塞。
update t_student set score = 100 where id = 20; 表中存在id=20的记录，这里会锁住20这个记录，不会有gap锁，其他事务此时向任意区间插入数据都不会被阻塞。
两个事务的间隙锁之间是相互兼容的，不会产生冲突。
在MySQL官网上还有一段非常关键的描述：
Gap locks in InnoDB are “purely inhibitive”, which means that their only purpose is to prevent other transactions from Inserting to the gap. Gap locks can co-exist. A gap lock taken by one transaction does not prevent another transaction from taking a gap lock on the same gap. There is no difference between shared and exclusive gap locks. They do not conflict with each other, and they perform the same function.
间隙锁的意义只在于阻止区间被插入，因此是可以共存的。一个事务获取的间隙锁不会阻止另一个事务获取同一个间隙范围的间隙锁，共享（S型）和排他（X型）的间隙锁是没有区别的，他们相互不冲突，且功能相同。
在MySQL的官方文档中有以下重要描述：
An Insert intention lock is a type of gap lock set by Insert operations prior to row Insertion. This lock signals the intent to Insert in such a way that multiple transactions Inserting into the same index gap need not wait for each other if they are not Inserting at the same position within the gap. Suppose that there are index records with values of 4 and 7. Separate transactions that attempt to Insert values of 5 and 6, respectively, each lock the gap between 4 and 7 with Insert intention locks prior to obtaining the exclusive lock on the Inserted row, but do not block each other because the rows are nonconflicting.
这段话表明尽管插入意向锁是一种特殊的间隙锁，但不同于间隙锁的是，该锁只用于并发插入操作。
如果说间隙锁锁住的是一个区间，那么「插入意向锁」锁住的就是一个点。因而从这个角度来说，插入意向锁确实是一种特殊的间隙锁。
插入意向锁与间隙锁的另一个非常重要的差别是：尽管「插入意向锁」也属于间隙锁，但两个事务却不能在同一时间内，一个拥有间隙锁，另一个拥有该间隙区间内的插入意向锁（当然，插入意向锁如果不在间隙锁区间内则是可以的）。所以，插入意向锁和间隙锁之间是冲突的。
另外，我补充一点，插入意向锁的生成时机：
每插入一条新记录，都需要看一下待插入记录的下一条记录上是否已经被加了间隙锁，如果已加间隙锁，此时会生成一个插入意向锁，然后锁的状态设置为等待状态（PS：MySQL 加锁时，是先生成锁结构，然后设置锁的状态，如果锁状态是等待状态，并不是意味着事务成功获取到了锁，只有当锁状态为正常状态时，才代表事务成功获取到了锁），现象就是 Insert 语句会被阻塞。
参考：https://blog.csdn.net/weixin_44681349/article/details/134898609
