行级别的 - Share and Exclusive Locks

• 共享锁 S：允许持有S锁的事务对行进行读操作
• 排他锁 X: 允许持有X锁的事务对行进行update或delete操作

表级别的意向锁 - Intention Lock

InnoDB支持多粒度的锁定，允许行锁和表锁共存。通过意向锁来实现。

比如，SELECT ... LOCK IN SHARE MODE sets an IS lock and SELECT ... FOR UPDATE sets an IX lock.

• 意向共享锁（IS）：在对数据行获取S锁之前，必须先获取IS锁或更强级别的锁
• 意向排他锁（IX）：在对数据行获取X锁之前，必须先获取IX锁

不同类型锁之间的兼容总结

A lock is granted to a requesting transaction if it is compatible with existing locks, but not if it conflicts with existing locks.

A transaction waits until the conflicting existing lock is released. If a lock request conflicts with an existing lock and cannotbe granted because it would cause deadlock, an error occurs.

因此，除了对整个表的请求（比如，LOCK TABLES ... WRITE）外，意向锁不会阻塞其他的事务。持有意向锁表示持有者正在锁定数据行，或者即将锁定数据行。

deadlock：

A deadlock can occur when the transactions lock rows in multiple tables (through statements such as UPDATE or SELECT ... FOR UPDATE), but in the opposite order. A deadlock can also occur when such statements lock ranges of index records and gaps, with each transaction acquiring some locks but not others due to a timing issue.

索引记录上的锁 - Record Lock

比如，SELECT c1 FOR UPDATE FROM t WHERE c1 = 10;避免其他的事务在t.c1=10的位置进行insert、update和delete操作

record lock始终锁定索引记录，即使一个表没有进行索引定义，对于这种情况，InnoDB创建一个隐藏的聚集索引并使用该索引记录锁定

Cap Lock

比如，SELECT c1 FOR UPDATE FROM t WHERE c1 BETWEEN 10 and 20;会在t.c1在10到20之间的索引记录上加锁，防止其他的事务在t.c1列上插入10到20之间的值

A next-key lock is a combination of a record lock on the index record and a gap lock on the gap before the index record.

InnoDB uses next-key locks for searches and index scans, which prevents phantom rows .

Insert Intention Locks

是gap锁的一种。多个事务如果不是在相同的索引范围内插入，则无需等待彼此

测试：

mysql> select * from t2;
+------+
| id   |
+------+
|    1 |
|    5 |
|    8 |
|   11 |
+------+

会话S1                                                         会话S2
mysql> begin;                                                                          
Query OK, 0 rows affected (0.00 sec)                                                   
                                                                                                                                                               
mysql> insert into t2(id) values(7);                 mysql> begin;                                                   
Query OK, 1 row affected (0.00 sec)               Query OK, 0 rows affected (0.00 sec)  

                                                                   mysql> insert into t2(id) values(6);
                                                                   Query OK, 1 row affected (0.00 sec)

                                                                   mysql> insert into t2(id) values(7);
                                                                   Query OK, 1 row affected (0.00 sec)

两个会话在插入的行上获取排他锁前，分别在id为5~8的行记录范围内持有intention locks，但不会阻塞对方，因为行之间没有冲突。

再举一个会话在要插入的行记录上获取排他锁之前获取insert intention lock但被阻塞的例子：

S1在id>5的行持有排它锁。此排它锁包含5~11之间的gap锁

mysql> begin;
Query OK, 0 rows affected (0.00 sec)

mysql> select * from t2 where id>5 for update;
+------+
| id   |
+------+
|    8 |
|   11 |
+------+

S2上在S1查询出的行范围内插入行记录。会持有insert intention lock，但是会等待排它锁

mysql> begin;
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t2 values(7);
ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction
mysql>

InnoDB 存储引擎的锁机制