InnoDB的行锁模式及加锁方法

妖狐艹你老母 2022-07-14 20:26 177阅读 0赞

InnoDB实现了以下两种类型的行锁。

l  共享锁（S）：允许一个事务去读一行，阻止其他事务获得相同数据集的排他锁。

l  排他锁（X)：允许获得排他锁的事务更新数据，阻止其他事务取得相同数据集的共享读锁和排他写锁。

另外，为了允许行锁和表锁共存，实现多粒度锁机制，InnoDB还有两种内部使用的意向锁（Intention Locks），这两种意向锁都是表锁。

l  意向共享锁（IS）：事务打算给数据行加行共享锁，事务在给一个数据行加共享锁前必须先取得该表的IS锁。

l  意向排他锁（IX）：事务打算给数据行加行排他锁，事务在给一个数据行加排他锁前必须先取得该表的IX锁。

上述锁模式的兼容情况具体如表20-6所示。

表20-6                                            InnoDB行锁模式兼容性列表

<table style="width: 581px; border-collapse: collapse; border-style: none;"> 
 <tbody> 
 <tr style="height: 65.9pt;"> 
 <td style="padding-bottom: 0cm; padding-left: 5.4pt; width: 176.4pt; padding-right: 5.4pt; height: 65.9pt; padding-top: 0cm; border: windowtext 1pt solid;"> 请求锁模式 &nbsp;&nbsp; 是否兼容 当前锁模式 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; height: 65.9pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> X </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; height: 65.9pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> IX </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; height: 65.9pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> S </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 70.65pt; padding-right: 5.4pt; height: 65.9pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> IS </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 176.4pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> X </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 冲突 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 冲突 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 冲突 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 70.65pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 冲突 </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 176.4pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> IX </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 冲突 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 兼容 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 冲突 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 70.65pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 兼容 </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 176.4pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> S </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 冲突 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 冲突 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 兼容 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 70.65pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 兼容 </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 176.4pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> IS </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 冲突 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 兼容 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 63pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 兼容 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 70.65pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 兼容 </td> 
 </tr> 
 </tbody> 
</table>

如果一个事务请求的锁模式与当前的锁兼容，InnoDB就将请求的锁授予该事务；反之，如果两者不兼容，该事务就要等待锁释放。

意向锁是InnoDB自动加的，不需用户干预。对于UPDATE、DELETE和INSERT语句，InnoDB会自动给涉及数据集加排他锁（X)；对于普通SELECT语句，InnoDB不会加任何锁；事务可以通过以下语句显示给记录集加共享锁或排他锁。

¡  共享锁（S）：SELECT \* FROM table\_name WHERE ... LOCK IN SHARE MODE。

¡  排他锁（X)：SELECT \* FROM table\_name WHERE ... FOR UPDATE。

用SELECT ... IN SHARE MODE获得共享锁，主要用在需要数据依存关系时来确认某行记录是否存在，并确保没有人对这个记录进行UPDATE或者DELETE操作。但是如果当前事务也需要对该记录进行更新操作，则很有可能造成死锁，对于锁定行记录后需要进行更新操作的应用，应该使用SELECT... FOR UPDATE方式获得排他锁。

在如表20-7所示的例子中，使用了SELECT ... IN SHARE MODE加锁后再更新记录，看看会出现什么情况，其中actor表的actor\_id字段为主键。

表20-7                     InnoDB存储引擎的共享锁例子

<table style="border-collapse: collapse; margin-left: 5.4pt; border-style: none;"> 
 <tbody> 
 <tr> 
 <td style="padding-bottom: 0cm; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; padding-top: 0cm; border: windowtext 1pt solid;"> session_1 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> session_2 </td> 
 </tr> 
 <tr style="height: 2cm;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; border-top-width: 1pt; height: 2cm; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql&gt; select actor_id,first_name,last_name from actor where actor_id = 178; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | LISA&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | MONROE&nbsp;&nbsp;&nbsp; | +----------+------------+-----------+ 1 row in set (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-top-style: none; height: 2cm; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql&gt; select actor_id,first_name,last_name from actor where actor_id = 178; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | LISA&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | MONROE&nbsp;&nbsp;&nbsp; | +----------+------------+-----------+ 1 row in set (0.00 sec) </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 当前session对actor_id=178的记录加share mode 的共享锁： mysql&gt; select actor_id,first_name,last_name from actor where actor_id = 178 lock in share mode; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | LISA&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | MONROE&nbsp;&nbsp;&nbsp; | +----------+------------+-----------+ 1 row in set (0.01 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 其他session仍然可以查询记录，并也可以对该记录加share mode的共享锁： mysql&gt; select actor_id,first_name,last_name from actor where actor_id = 178 lock in share mode; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | LISA&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | MONROE&nbsp;&nbsp;&nbsp; | +----------+------------+-----------+ 1 row in set (0.01 sec) </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 当前session对锁定的记录进行更新操作，等待锁： mysql&gt; update actor set last_name = 'MONROE T' where actor_id = 178; 等待 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 其他session也对该记录进行更新操作，则会导致死锁退出： mysql&gt; update actor set last_name = 'MONROE T' where actor_id = 178; ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 获得锁后，可以成功更新： mysql&gt; update actor set last_name = 'MONROE T' where actor_id = 178; Query OK, 1 row affected (17.67 sec) Rows matched: 1&nbsp; Changed: 1&nbsp; Warnings: 0 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 </tr> 
 </tbody> 
</table>

当使用SELECT...FOR UPDATE加锁后再更新记录，出现如表20-8所示的情况。

表20-8                               InnoDB存储引擎的排他锁例子

<table style="border-collapse: collapse; margin-left: 5.4pt; border-style: none;"> 
 <tbody> 
 <tr> 
 <td style="padding-bottom: 0cm; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; padding-top: 0cm; border: windowtext 1pt solid;"> session_1 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> session_2 </td> 
 </tr> 
 <tr style="height: 2cm;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; border-top-width: 1pt; height: 2cm; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql&gt; select actor_id,first_name,last_name from actor where actor_id = 178; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | LISA&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | MONROE&nbsp;&nbsp;&nbsp; | +----------+------------+-----------+ 1 row in set (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-top-style: none; height: 2cm; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql&gt; select actor_id,first_name,last_name from actor where actor_id = 178; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | LISA&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | MONROE&nbsp;&nbsp;&nbsp; | +----------+------------+-----------+ 1 row in set (0.00 sec) </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 当前session对actor_id=178的记录加for update的共享锁： mysql&gt; select actor_id,first_name,last_name from actor where actor_id = 178 for update; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | LISA&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | MONROE&nbsp;&nbsp;&nbsp; | +----------+------------+-----------+ 1 row in set (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 其他session可以查询该记录，但是不能对该记录加共享锁，会等待获得锁： mysql&gt; select actor_id,first_name,last_name from actor where actor_id = 178; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | LISA&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | MONROE&nbsp;&nbsp;&nbsp; | +----------+------------+-----------+ 1 row in set (0.00 sec) mysql&gt; select actor_id,first_name,last_name from actor where actor_id = 178 for update; 等待 </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 当前session可以对锁定的记录进行更新操作，更新后释放锁： mysql&gt; update actor set last_name = 'MONROE T' where actor_id = 178; Query OK, 1 row affected (0.00 sec) Rows matched: 1&nbsp; Changed: 1&nbsp; Warnings: 0 mysql&gt; commit; Query OK, 0 rows affected (0.01 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 207pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 其他session获得锁，得到其他session提交的记录： mysql&gt; select actor_id,first_name,last_name from actor where actor_id = 178 for update; +----------+------------+-----------+ | actor_id | first_name | last_name | +----------+------------+-----------+ | 178&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | LISA&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | MONROE T&nbsp; | +----------+------------+-----------+ 1 row in set (9.59 sec) </td> 
 </tr> 
 </tbody> 
</table>

### InnoDB行锁实现方式 ###

InnoDB行锁是通过给索引上的索引项加锁来实现的，这一点MySQL与Oracle不同，后者是通过在数据块中对相应数据行加锁来实现的。InnoDB这种行锁实现特点意味着：只有通过索引条件检索数据，InnoDB才使用行级锁，否则，InnoDB将使用表锁！

在实际应用中，要特别注意InnoDB行锁的这一特性，不然的话，可能导致大量的锁冲突，从而影响并发性能。下面通过一些实际例子来加以说明。

（1）在不通过索引条件查询的时候，InnoDB确实使用的是表锁，而不是行锁。

在如表20-9所示的例子中，开始tab\_no\_index表没有索引：

mysql> create table tab\_no\_index(id int,name varchar(10)) engine=innodb;

Query OK, 0 rows affected (0.15 sec)

mysql> insert into tab\_no\_index values(1,'1'),(2,'2'),(3,'3'),(4,'4');

Query OK, 4 rows affected (0.00 sec)

Records: 4  Duplicates: 0  Warnings: 0

表20-9               InnoDB存储引擎的表在不使用索引时使用表锁例子

<table style="border-collapse: collapse; margin-left: 5.4pt; border-style: none;"> 
 <tbody> 
 <tr> 
 <td style="padding-bottom: 0cm; padding-left: 5.4pt; width: 219pt; padding-right: 5.4pt; padding-top: 0cm; border: windowtext 1pt solid;"> session_1 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 195pt; padding-right: 5.4pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> session_2 </td> 
 </tr> 
 <tr style="height: 2cm;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 219pt; padding-right: 5.4pt; border-top-width: 1pt; height: 2cm; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; set autocommit=0; Query OK, 0 rows affected (0.00 sec) mysql&gt; select * from tab_no_index where id = 1 ; +------+------+ | id&nbsp;&nbsp; | name | +------+------+ | 1&nbsp;&nbsp;&nbsp; | 1&nbsp;&nbsp;&nbsp; | +------+------+ 1 row in set (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 195pt; padding-right: 5.4pt; border-top-style: none; height: 2cm; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; set autocommit=0; Query OK, 0 rows affected (0.00 sec) mysql&gt; select * from tab_no_index where id = 2 ; +------+------+ | id&nbsp;&nbsp; | name | +------+------+ | 2&nbsp;&nbsp;&nbsp; | 2&nbsp;&nbsp;&nbsp; | +------+------+ 1 row in set (0.00 sec) </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 219pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; select * from tab_no_index where id = 1 for update; +------+------+ | id&nbsp;&nbsp; | name | +------+------+ | 1&nbsp;&nbsp;&nbsp; | 1&nbsp;&nbsp;&nbsp; | +------+------+ 1 row in set (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 195pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 219pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 195pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; select * from tab_no_index where id = 2 for update; 等待 </td> 
 </tr> 
 </tbody> 
</table>

在如表20-9所示的例子中，看起来session\_1只给一行加了排他锁，但session\_2在请求其他行的排他锁时，却出现了锁等待！原因就是在没有索引的情况下，InnoDB只能使用表锁。当我们给其增加一个索引后，InnoDB就只锁定了符合条件的行，如表20-10所示。

创建tab\_with\_index表，id字段有普通索引：

mysql> create table tab\_with\_index(id int,name varchar(10)) engine=innodb;

Query OK, 0 rows affected (0.15 sec)

mysql> alter table tab\_with\_index add index id(id);

Query OK, 4 rows affected (0.24 sec)

Records: 4  Duplicates: 0  Warnings: 0

表20-10                                InnoDB存储引擎的表在使用索引时使用行锁例子

（2）由于MySQL的行锁是针对索引加的锁，不是针对记录加的锁，所以虽然是访问不同行的记录，但是如果是使用相同的索引键，是会出现锁冲突的。应用设计的时候要注意这一点。

在如表20-11所示的例子中，表tab\_with\_index的id字段有索引，name字段没有索引：

mysql> alter table tab\_with\_index drop index name;

Query OK, 4 rows affected (0.22 sec)

Records: 4  Duplicates: 0  Warnings: 0

mysql> insert into tab\_with\_index  values(1,'4');

Query OK, 1 row affected (0.00 sec)

mysql> select \* from tab\_with\_index where id = 1;

\+------+------+

| id   | name |

\+------+------+

| 1    | 1    |

| 1    | 4    |

\+------+------+

2 rows in set (0.00 sec)

表20-11                InnoDB存储引擎使用相同索引键的阻塞例子

<table style="width: 548px; border-collapse: collapse; margin-left: 5.4pt; border-style: none;"> 
 <tbody> 
 <tr> 
 <td style="padding-bottom: 0cm; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; padding-top: 0cm; border: windowtext 1pt solid;"> session_1 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> session_2 </td> 
 </tr> 
 <tr style="height: 29.75pt;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-width: 1pt; height: 29.75pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; set autocommit=0; Query OK, 0 rows affected (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-style: none; height: 29.75pt; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; set autocommit=0; Query OK, 0 rows affected (0.00 sec) </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; select * from tab_with_index where id = 1 and name = '1' for update; +------+------+ | id&nbsp;&nbsp; | name | +------+------+ | 1&nbsp;&nbsp;&nbsp; | 1&nbsp;&nbsp;&nbsp; | +------+------+ 1 row in set (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 虽然session_2访问的是和session_1不同的记录，但是因为使用了相同的索引，所以需要等待锁： mysql&gt; select * from tab_with_index where id = 1 and name = '4' for update; 等待 </td> 
 </tr> 
 </tbody> 
</table>

（3）当表有多个索引的时候，不同的事务可以使用不同的索引锁定不同的行，另外，不论是使用主键索引、唯一索引或普通索引，InnoDB都会使用行锁来对数据加锁。

在如表20-12所示的例子中，表tab\_with\_index的id字段有主键索引，name字段有普通索引：

mysql> alter table tab\_with\_index add index name(name);

Query OK, 5 rows affected (0.23 sec)

Records: 5  Duplicates: 0  Warnings: 0

表20-12                                  InnoDB存储引擎的表使用不同索引的阻塞例子

<table style="border-collapse: collapse; margin-left: 5.4pt; border-style: none;"> 
 <tbody> 
 <tr> 
 <td style="padding-bottom: 0cm; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; padding-top: 0cm; border: windowtext 1pt solid;"> session_1 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> session_2 </td> 
 </tr> 
 <tr style="height: 28.25pt;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-width: 1pt; height: 28.25pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; set autocommit=0; Query OK, 0 rows affected (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-style: none; height: 28.25pt; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; set autocommit=0; Query OK, 0 rows affected (0.00 sec) </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; select * from tab_with_index where id = 1 for update; +------+------+ | id &nbsp;&nbsp;| name | +------+------+ | 1&nbsp;&nbsp;&nbsp; | 1&nbsp;&nbsp;&nbsp; | | 1&nbsp;&nbsp;&nbsp; | 4&nbsp;&nbsp;&nbsp; | +------+------+ 2 rows in set (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Session_2使用name的索引访问记录，因为记录没有被索引，所以可以获得锁： mysql&gt; select * from tab_with_index where name = '2' for update; +------+------+ | id&nbsp;&nbsp; | name | +------+------+ | 2&nbsp;&nbsp;&nbsp; | 2&nbsp;&nbsp;&nbsp; | +------+------+ 1 row in set (0.00 sec) </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; padding-bottom: 0cm; border-top-color: windowtext; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 205.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 由于访问的记录已经被session_1锁定，所以等待获得锁。： mysql&gt; select * from tab_with_index where name = '4' for update; </td> 
 </tr> 
 </tbody> 
</table>

（4）即便在条件中使用了索引字段，但是否使用索引来检索数据是由MySQL通过判断不同执行计划的代价来决定的，如果MySQL认为全表扫描效率更高，比如对一些很小的表，它就不会使用索引，这种情况下InnoDB将使用表锁，而不是行锁。因此，在分析锁冲突时，别忘了检查SQL的执行计划，以确认是否真正使用了索引。关于MySQL在什么情况下不使用索引的详细讨论，参见本章“索引问题”一节的介绍。

在下面的例子中，检索值的数据类型与索引字段不同，虽然MySQL能够进行数据类型转换，但却不会使用索引，从而导致InnoDB使用表锁。通过用explain检查两条SQL的执行计划，我们可以清楚地看到了这一点。

例子中tab\_with\_index表的name字段有索引，但是name字段是varchar类型的，如果where条件中不是和varchar类型进行比较，则会对name进行类型转换，而执行的全表扫描。

mysql> alter table tab\_no\_index add index name(name);

Query OK, 4 rows affected (8.06 sec)

Records: 4  Duplicates: 0  Warnings: 0

mysql> explain select \* from tab\_with\_index where name = 1 /G

\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\* 1. row \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*

id: 1

select\_type: SIMPLE

table: tab\_with\_index

type: ALL

possible\_keys: name