[MySQL] InnoDB锁问题

秒速五厘米 2022-07-10 09:28 132阅读 0赞

摘自： [http://book.csdn.net/bookfiles/667/10066721021.shtml][http_book.csdn.net_bookfiles_667_10066721021.shtml]

InnoDB与MyISAM的最大不同有两点：一是支持事务（TRANSACTION）；二是采用了行级锁。行级锁与表级锁本来就有许多不同之处，另外，事务的引入也带来了一些新问题。下面我们先介绍一点背景知识，然后详细讨论InnoDB的锁问题。

### 背景知识 ###

#### 1．事务（Transaction）及其ACID属性 ####

事务是由一组SQL语句组成的逻辑处理单元，事务具有以下4个属性，通常简称为事务的ACID属性。

l         原子性（Atomicity）：事务是一个原子操作单元，其对数据的修改，要么全都执行，要么全都不执行。

l         一致性（Consistent）：在事务开始和完成时，数据都必须保持一致状态。这意味着所有相关的数据规则都必须应用于事务的修改，以保持数据的完整性；事务结束时，所有的内部数据结构（如B树索引或双向链表）也都必须是正确的。

l         隔离性（Isolation）：数据库系统提供一定的隔离机制，保证事务在不受外部并发操作影响的“独立”环境执行。这意味着事务处理过程中的中间状态对外部是不可见的，反之亦然。

l         持久性（Durable）：事务完成之后，它对于数据的修改是永久性的，即使出现系统故障也能够保持。

银行转帐就是事务的一个典型例子。

#### 2．并发事务处理带来的问题 ####

相对于串行处理来说，并发事务处理能大大增加数据库资源的利用率，提高数据库系统的事务吞吐量，从而可以支持更多的用户。但并发事务处理也会带来一些问题，主要包括以下几种情况。

l  更新丢失（Lost Update）：当两个或多个事务选择同一行，然后基于最初选定的值更新该行时，由于每个事务都不知道其他事务的存在，就会发生丢失更新问题－－最后的更新覆盖了由其他事务所做的更新。例如，两个编辑人员制作了同一文档的电子副本。每个编辑人员独立地更改其副本，然后保存更改后的副本，这样就覆盖了原始文档。最后保存其更改副本的编辑人员覆盖另一个编辑人员所做的更改。如果在一个编辑人员完成并提交事务之前，另一个编辑人员不能访问同一文件，则可避免此问题。

l  脏读（Dirty Reads）：一个事务正在对一条记录做修改，在这个事务完成并提交前，这条记录的数据就处于不一致状态；这时，另一个事务也来读取同一条记录，如果不加控制，第二个事务读取了这些“脏”数据，并据此做进一步的处理，就会产生未提交的数据依赖关系。这种现象被形象地叫做"脏读"。

l  不可重复读（Non-Repeatable Reads）：一个事务在读取某些数据后的某个时间，再次读取以前读过的数据，却发现其读出的数据已经发生了改变、或某些记录已经被删除了！这种现象就叫做“不可重复读”。

l  幻读（Phantom Reads）：一个事务按相同的查询条件重新读取以前检索过的数据，却发现其他事务插入了满足其查询条件的新数据，这种现象就称为“幻读”。

#### 3．事务隔离级别 ####

在上面讲到的并发事务处理带来的问题中，“更新丢失”通常是应该完全避免的。但防止更新丢失，并不能单靠数据库事务控制器来解决，需要应用程序对要更新的数据加必要的锁来解决，因此，防止更新丢失应该是应用的责任。

“脏读”、“不可重复读”和“幻读”，其实都是数据库读一致性问题，必须由数据库提供一定的事务隔离机制来解决。数据库实现事务隔离的方式，基本上可分为以下两种。

l  一种是在读取数据前，对其加锁，阻止其他事务对数据进行修改。

l  另一种是不用加任何锁，通过一定机制生成一个数据请求时间点的一致性数据快照（Snapshot)，并用这个快照来提供一定级别（语句级或事务级）的一致性读取。从用户的角度来看，好象是数据库可以提供同一数据的多个版本，因此，这种技术叫做数据多版本并发控制（MultiVersion Concurrency Control，简称MVCC或MCC），也经常称为多版本数据库。

数据库的事务隔离越严格，并发副作用越小，但付出的代价也就越大，因为事务隔离实质上就是使事务在一定程度上“串行化”进行，这显然与“并发”是矛盾的。同时，不同的应用对读一致性和事务隔离程度的要求也是不同的，比如许多应用对“不可重复读”和“幻读”并不敏感，可能更关心数据并发访问的能力。

为了解决“隔离”与“并发”的矛盾，ISO/ANSI SQL92定义了4个事务隔离级别，每个级别的隔离程度不同，允许出现的副作用也不同，应用可以根据自己的业务逻辑要求，通过选择不同的隔离级别来平衡“隔离”与“并发”的矛盾。表20-5很好地概括了这4个隔离级别的特性。

表20-5                                             4种隔离级别比较

<table style="width: 539px; border-collapse: collapse; margin-left: 5.4pt; border-style: none;"> 
 <tbody> 
 <tr style="height: 63.95pt;"> 
 <td style="padding-bottom: 0cm; padding-left: 5.4pt; width: 151.65pt; padding-right: 5.4pt; height: 63.95pt; padding-top: 0cm; border: windowtext 1pt solid;"> 读数据一致性及允许的并发副作用 隔离级别 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 110.6pt; padding-right: 5.4pt; height: 63.95pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 读数据一致性 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 42.55pt; padding-right: 5.4pt; height: 63.95pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 脏读 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 2cm; padding-right: 5.4pt; height: 63.95pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 不可重复读 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 42.5pt; padding-right: 5.4pt; height: 63.95pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 幻读 </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: 151.65pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 未提交读（Read uncommitted） </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 110.6pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 最低级别，只能保证不读取物理上损坏的数据 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 42.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 是 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 2cm; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 是 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 42.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 是 </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: 151.65pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 已提交度（Read committed） </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 110.6pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 语句级 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 42.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 否 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 2cm; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 是 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 42.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 是 </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: 151.65pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 可重复读（Repeatable read） </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 110.6pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 事务级 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 42.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 否 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 2cm; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 否 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 42.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 是 </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: 151.65pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 可序列化（Serializable） </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 110.6pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 最高级别，事务级 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 42.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 否 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 2cm; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 否 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 42.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 否 </td> 
 </tr> 
 </tbody> 
</table>

最后要说明的是：各具体数据库并不一定完全实现了上述4个隔离级别，例如，Oracle只提供Read committed和Serializable两个标准隔离级别，另外还提供自己定义的Read only隔离级别；SQL Server除支持上述ISO/ANSI SQL92定义的4个隔离级别外，还支持一个叫做“快照”的隔离级别，但严格来说它是一个用MVCC实现的Serializable隔离级别。MySQL支持全部4个隔离级别，但在具体实现时，有一些特点，比如在一些隔离级别下是采用MVCC一致性读，但某些情况下又不是，这些内容在后面的章节中将会做进一步介绍。

### 获取InnoDB行锁争用情况     ###

可以通过检查InnoDB\_row\_lock状态变量来分析系统上的行锁的争夺情况：

mysql> show status like 'innodb\_row\_lock%';

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

| Variable\_name                 | Value |

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

| InnoDB\_row\_lock\_current\_waits | 0     |

| InnoDB\_row\_lock\_time          | 0     |

| InnoDB\_row\_lock\_time\_avg      | 0     |

| InnoDB\_row\_lock\_time\_max      | 0     |

| InnoDB\_row\_lock\_waits         | 0     |

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

5 rows in set (0.01 sec)

如果发现锁争用比较严重，如InnoDB\_row\_lock\_waits和InnoDB\_row\_lock\_time\_avg的值比较高，还可以通过设置InnoDB Monitors来进一步观察发生锁冲突的表、数据行等，并分析锁争用的原因。

具体方法如下：

mysql> CREATE TABLE innodb\_monitor(a INT) ENGINE=INNODB;

Query OK, 0 rows affected (0.14 sec)

然后就可以用下面的语句来进行查看：

mysql> Show innodb status/G;

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

Type: InnoDB

Name:

Status:

…

\------------

TRANSACTIONS

\------------

Trx id counter 0 117472192

Purge done for trx's n:o < 0 117472190 undo n:o < 0 0

History list length 17

Total number of lock structs in row lock hash table 0

LIST OF TRANSACTIONS FOR EACH SESSION:

\---TRANSACTION 0 117472185, not started, process no 11052, OS thread id 1158191456

MySQL thread id 200610, query id 291197 localhost root

\---TRANSACTION 0 117472183, not started, process no 11052, OS thread id 1158723936

MySQL thread id 199285, query id 291199 localhost root

Show innodb status

…

监视器可以通过发出下列语句来停止查看：

mysql> DROP TABLE innodb\_monitor;

Query OK, 0 rows affected (0.05 sec)

设置监视器后，在SHOW INNODB STATUS的显示内容中，会有详细的当前锁等待的信息，包括表名、锁类型、锁定记录的情况等，便于进行进一步的分析和问题的确定。打开监视器以后，默认情况下每15秒会向日志中记录监控的内容，如果长时间打开会导致.err文件变得非常的巨大，所以用户在确认问题原因之后，要记得删除监控表以关闭监视器，或者通过使用“--console”选项来启动服务器以关闭写日志文件。

### InnoDB的行锁模式及加锁方法 ###

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

key: NULL

key\_len: NULL

ref: NULL

rows: 4

Extra: Using where

1 row in set (0.00 sec)

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

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

id: 1

select\_type: SIMPLE

table: tab\_with\_index

type: ref

possible\_keys: name

key: name

key\_len: 23

ref: const

rows: 1

Extra: Using where

1 row in set (0.00 sec)

### 间隙锁（Next-Key锁） ###

当我们用范围条件而不是相等条件检索数据，并请求共享或排他锁时，InnoDB会给符合条件的已有数据记录的索引项加锁；对于键值在条件范围内但并不存在的记录，叫做“间隙（GAP)”，InnoDB也会对这个“间隙”加锁，这种锁机制就是所谓的间隙锁（Next-Key锁）。

举例来说，假如emp表中只有101条记录，其empid的值分别是 1,2,...,100,101，下面的SQL：

Select \* from  emp where empid > 100 for update;

是一个范围条件的检索，InnoDB不仅会对符合条件的empid值为101的记录加锁，也会对empid大于101（这些记录并不存在）的“间隙”加锁。

InnoDB使用间隙锁的目的，一方面是为了防止幻读，以满足相关隔离级别的要求，对于上面的例子，要是不使用间隙锁，如果其他事务插入了empid大于100的任何记录，那么本事务如果再次执行上述语句，就会发生幻读；另外一方面，是为了满足其恢复和复制的需要。有关其恢复和复制对锁机制的影响，以及不同隔离级别下InnoDB使用间隙锁的情况，在后续的章节中会做进一步介绍。

很显然，在使用范围条件检索并锁定记录时，InnoDB这种加锁机制会阻塞符合条件范围内键值的并发插入，这往往会造成严重的锁等待。因此，在实际应用开发中，尤其是并发插入比较多的应用，我们要尽量优化业务逻辑，尽量使用相等条件来访问更新数据，避免使用范围条件。

还要特别说明的是，InnoDB除了通过范围条件加锁时使用间隙锁外，如果使用相等条件请求给一个不存在的记录加锁，InnoDB也会使用间隙锁！

在如表20-13所示的例子中，假如emp表中只有101条记录，其empid的值分别是1,2,......,100,101。

表20-13                InnoDB存储引擎的间隙锁阻塞例子

<table style="border-collapse: collapse; margin-left: 5.4pt; border-style: none;"> 
 <tbody> 
 <tr> 
 <td style="padding-bottom: 0cm; padding-left: 5.4pt; width: 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; select @@tx_isolation; +-----------------+ | @@tx_isolation&nbsp; | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) 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: 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; select @@tx_isolation; +-----------------+ | @@tx_isolation&nbsp; | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) 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: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 当前session对不存在的记录加for update的锁： mysql&gt; select * from emp where empid = 102 for update; Empty 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;"> 这时，如果其他session插入empid为201的记录（注意：这条记录并不存在），也会出现锁等待： mysql&gt;insert into emp(empid,...) values(201,...); 阻塞等待 </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;"> Session_1 执行rollback： mysql&gt; rollback; Query OK, 0 rows affected (13.04 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;"> 由于其他session_1回退后释放了Next-Key锁，当前session可以获得锁并成功插入记录： mysql&gt;insert into emp(empid,...) values(201,...); Query OK, 1 row affected (13.35 sec) </td> 
 </tr> 
 </tbody> 
</table>

### 恢复和复制的需要，对InnoDB锁机制的影响 ###

MySQL通过BINLOG录执行成功的INSERT、UPDATE、DELETE等更新数据的SQL语句，并由此实现MySQL数据库的恢复和主从复制（可以参见本书“管理篇”的介绍）。MySQL的恢复机制（复制其实就是在Slave Mysql不断做基于BINLOG的恢复）有以下特点。

l  一是MySQL的恢复是SQL语句级的，也就是重新执行BINLOG中的SQL语句。这与Oracle数据库不同，Oracle是基于数据库文件块的。

l  二是MySQL的Binlog是按照事务提交的先后顺序记录的，恢复也是按这个顺序进行的。这点也与Oralce不同，Oracle是按照系统更新号（System Change Number，SCN）来恢复数据的，每个事务开始时，Oracle都会分配一个全局唯一的SCN，SCN的顺序与事务开始的时间顺序是一致的。

从上面两点可知，MySQL的恢复机制要求：在一个事务未提交前，其他并发事务不能插入满足其锁定条件的任何记录，也就是不允许出现幻读，这已经超过了ISO/ANSI SQL92“可重复读”隔离级别的要求，实际上是要求事务要串行化。这也是许多情况下，InnoDB要用到间隙锁的原因，比如在用范围条件更新记录时，无论在Read Commited或是Repeatable Read隔离级别下，InnoDB都要使用间隙锁，但这并不是隔离级别要求的，有关InnoDB在不同隔离级别下加锁的差异在下一小节还会介绍。

另外，对于“insert  into target\_tab select \* from source\_tab where ...”和“create  table new\_tab ...select ... From  source\_tab where ...(CTAS)”这种SQL语句，用户并没有对source\_tab做任何更新操作，但MySQL对这种SQL语句做了特别处理。先来看如表20-14的例子。

表20-14                   CTAS操作给原表加锁例子

<table style="border-collapse: collapse; margin-left: 9.25pt; border-style: none;"> 
 <tbody> 
 <tr style="height: 31.3pt;"> 
 <td style="width: 182.2pt; height: 31.3pt; border: windowtext 1pt solid; padding: 0cm;"> session_1 </td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; height: 31.3pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding: 0cm;"> session_2 </td> 
 </tr> 
 <tr style="page-break-inside: avoid; height: 2cm;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 182.2pt; border-top-width: 1pt; height: 2cm; border-right: windowtext 1pt solid; padding: 0cm;"> mysql&gt; set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql&gt; select * from target_tab; Empty set (0.00 sec) mysql&gt; select * from source_tab where name = '1'; +----+------+----+ | d1 | name | d2 | +----+------+----+ |&nbsp; 4 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 5 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 6 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 7 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 8 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | +----+------+----+ 5 rows in set (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; height: 2cm; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;"> mysql&gt; set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql&gt; select * from target_tab; Empty set (0.00 sec) mysql&gt; select * from source_tab where name = '1'; +----+------+----+ | d1 | name | d2 | +----+------+----+ |&nbsp; 4 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 5 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 6 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 7 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 8 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | +----+------+----+ 5 rows in set (0.00 sec) </td> 
 </tr> 
 <tr style="page-break-inside: avoid;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 182.2pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding: 0cm;"> mysql&gt; insert into target_tab select d1,name from source_tab where name = '1'; Query OK, 5 rows affected (0.00 sec) Records: 5&nbsp; Duplicates: 0&nbsp; Warnings: 0 </td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;">&nbsp;</td> 
 </tr> 
 <tr style="page-break-inside: avoid; height: 41.75pt;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 182.2pt; border-top-width: 1pt; height: 41.75pt; border-right: windowtext 1pt solid; padding: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; height: 41.75pt; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;"> mysql&gt; update source_tab set name = '1' where name = '8'; 等待 </td> 
 </tr> 
 <tr style="page-break-inside: avoid; height: 13.1pt;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 182.2pt; border-top-width: 1pt; height: 13.1pt; border-right: windowtext 1pt solid; padding: 0cm;"> commit; </td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; height: 13.1pt; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;">&nbsp;</td> 
 </tr> 
 <tr style="page-break-inside: avoid; height: 18.75pt;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 182.2pt; border-top-width: 1pt; height: 18.75pt; border-right: windowtext 1pt solid; padding: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; height: 18.75pt; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;"> 返回结果 commit; </td> 
 </tr> 
 </tbody> 
</table>

在上面的例子中，只是简单地读source\_tab表的数据，相当于执行一个普通的SELECT语句，用一致性读就可以了。ORACLE正是这么做的，它通过MVCC技术实现的多版本数据来实现一致性读，不需要给source\_tab加任何锁。我们知道InnoDB也实现了多版本数据，对普通的SELECT一致性读，也不需要加任何锁；但这里InnoDB却给source\_tab加了共享锁，并没有使用多版本数据一致性读技术！

MySQL为什么要这么做呢？其原因还是为了保证恢复和复制的正确性。因为不加锁的话，如果在上述语句执行过程中，其他事务对source\_tab做了更新操作，就可能导致数据恢复的结果错误。为了演示这一点，我们再重复一下前面的例子，不同的是在session\_1执行事务前，先将系统变量innodb\_locks\_unsafe\_for\_binlog的值设置为“on”（其默认值为off），具体结果如表20-15所示。

表20-15                   CTAS操作不给原表加锁带来的安全问题例子

<table style="border-collapse: collapse; margin-left: 9.25pt; border-style: none;"> 
 <tbody> 
 <tr style="height: 18.95pt;"> 
 <td style="width: 182.2pt; height: 18.95pt; border: windowtext 1pt solid; padding: 0cm;"> session_1 </td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; height: 18.95pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding: 0cm;"> session_2 </td> 
 </tr> 
 <tr style="height: 56.65pt;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 182.2pt; border-top-width: 1pt; height: 56.65pt; border-right: windowtext 1pt solid; padding: 0cm;"> mysql&gt; set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql&gt;set innodb_locks_unsafe_for_binlog='on' Query OK, 0 rows affected (0.00 sec) mysql&gt; select * from target_tab; Empty set (0.00 sec) mysql&gt; select * from source_tab where name = '1'; +----+------+----+ | d1 | name | d2 | +----+------+----+ |&nbsp; 4 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 5 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 6 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 7 | 1&nbsp;&nbsp; &nbsp;|&nbsp; 1 | |&nbsp; 8 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | +----+------+----+ 5 rows in set (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; height: 56.65pt; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;"> mysql&gt; set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql&gt; select * from target_tab; Empty set (0.00 sec) mysql&gt; select * from source_tab where name = '1'; +----+------+----+ | d1 | name | d2 | +----+------+----+ |&nbsp; 4 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 5 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 6 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 7 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 8 | 1&nbsp;&nbsp;&nbsp; |&nbsp; 1 | +----+------+----+ 5 rows in set (0.00 sec) </td> 
 </tr> 
 <tr style="page-break-inside: avoid;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 182.2pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding: 0cm;"> mysql&gt; insert into target_tab select d1,name from source_tab where name = '1'; Query OK, 5 rows affected (0.00 sec) Records: 5&nbsp; Duplicates: 0&nbsp; Warnings: 0 </td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;">&nbsp;</td> 
 </tr> 
 <tr style="page-break-inside: avoid;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 182.2pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;"> session_1未提交，可以对session_1的select的记录进行更新操作。 mysql&gt; update source_tab set name = '8' where name = '1'; Query OK, 5 rows affected (0.00 sec) Rows matched: 5&nbsp; Changed: 5&nbsp; Warnings: 0 mysql&gt; select * from source_tab where name = '8'; +----+------+----+ | d1 | name | d2 | +----+------+----+ |&nbsp; 4 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 5 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 6 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 7 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 8 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | +----+------+----+ 5 rows in set (0.00 sec) </td> 
 </tr> 
 <tr style="page-break-inside: avoid;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 182.2pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;"> 更新操作先提交 mysql&gt; commit; Query OK, 0 rows affected (0.05 sec) </td> 
 </tr> 
 <tr style="page-break-inside: avoid;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 182.2pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding: 0cm;"> 插入操作后提交 mysql&gt; commit; Query OK, 0 rows affected (0.07 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;">&nbsp;</td> 
 </tr> 
 <tr style="page-break-inside: avoid;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 182.2pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding: 0cm;"> 此时查看数据，target_tab中可以插入source_tab更新前的结果，这符合应用逻辑： mysql&gt; select * from source_tab where name = '8'; +----+------+----+ | d1 | name | d2 | +----+------+----+ |&nbsp; 4 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 5 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 6 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 7 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 8 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | +----+------+----+ 5 rows in set (0.00 sec) mysql&gt; select * from target_tab; +------+------+ | id&nbsp;&nbsp; | name | +------+------+ | 4&nbsp;&nbsp;&nbsp; | 1.00 | | 5&nbsp;&nbsp;&nbsp; | 1.00 | | 6&nbsp;&nbsp;&nbsp; | 1.00 | | 7&nbsp;&nbsp;&nbsp; | 1.00 | | 8&nbsp;&nbsp;&nbsp; | 1.00 | +------+------+ 5 rows in set (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;"> mysql&gt; select * from tt1 where name = '1'; Empty set (0.00 sec) mysql&gt; select * from source_tab where name = '8'; +----+------+----+ | d1 | name | d2 | +----+------+----+ |&nbsp; 4 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 5 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 6 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 7 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | |&nbsp; 8 | 8&nbsp;&nbsp;&nbsp; |&nbsp; 1 | +----+------+----+ 5 rows in set (0.00 sec) mysql&gt; select * from target_tab; +------+------+ | id&nbsp;&nbsp; | name | +------+------+ | 4&nbsp;&nbsp;&nbsp; | 1.00 | | 5&nbsp;&nbsp;&nbsp; | 1.00 | | 6&nbsp;&nbsp;&nbsp; | 1.00 | | 7&nbsp;&nbsp;&nbsp; | 1.00 | | 8&nbsp;&nbsp;&nbsp; | 1.00 | +------+------+ 5 rows in set (0.00 sec) </td> 
 </tr> 
 </tbody> 
</table>

从上可见，设置系统变量innodb\_locks\_unsafe\_for\_binlog的值为“on”后，InnoDB不再对source\_tab加锁，结果也符合应用逻辑，但是如果分析BINLOG的内容：

......

SET TIMESTAMP=1169175130;

BEGIN;

\# at 274

\#070119 10:51:57 server id 1  end\_log\_pos 105   Query   thread\_id=1     exec\_time=0     error\_code=0

SET TIMESTAMP=1169175117;

update source\_tab set name = '8' where name = '1';

\# at 379

\#070119 10:52:10 server id 1  end\_log\_pos 406   Xid = 5

COMMIT;

\# at 406

\#070119 10:52:14 server id 1  end\_log\_pos 474   Query   thread\_id=2     exec\_time=0     error\_code=0

SET TIMESTAMP=1169175134;

BEGIN;

\# at 474

\#070119 10:51:29 server id 1  end\_log\_pos 119   Query   thread\_id=2     exec\_time=0     error\_code=0

SET TIMESTAMP=1169175089;

insert into target\_tab select d1,name from source\_tab where name = '1';

\# at 593

\#070119 10:52:14 server id 1  end\_log\_pos 620   Xid = 7

COMMIT;

......

可以发现，在BINLOG中，更新操作的位置在INSERT...SELECT之前，如果使用这个BINLOG进行数据库恢复，恢复的结果与实际的应用逻辑不符；如果进行复制，就会导致主从数据库不一致！

通过上面的例子，我们就不难理解为什么MySQL在处理“Insert  into target\_tab select \* from source\_tab where ...”和“create  table new\_tab ...select ... From  source\_tab where ...”时要给source\_tab加锁，而不是使用对并发影响最小的多版本数据来实现一致性读。还要特别说明的是，如果上述语句的SELECT是范围条件，InnoDB还会给源表加间隙锁（Next-Lock）。

因此，INSERT...SELECT...和CREATE TABLE...SELECT...语句，可能会阻止对源表的并发更新，造成对源表锁的等待。如果查询比较复杂的话，会造成严重的性能问题，我们在应用中应尽量避免使用。实际上，MySQL将这种SQL叫作不确定（non-deterministic）的SQL，不推荐使用。

如果应用中一定要用这种SQL来实现业务逻辑，又不希望对源表的并发更新产生影响，可以采取以下两种措施：

¡  一是采取上面示例中的做法，将innodb\_locks\_unsafe\_for\_binlog的值设置为“on”，强制MySQL使用多版本数据一致性读。但付出的代价是可能无法用binlog正确地恢复或复制数据，因此，不推荐使用这种方式。

¡  二是通过使用“select \* from source\_tab ... Into outfile”和“load data infile ...”语句组合来间接实现，采用这种方式MySQL不会给source\_tab加锁。

### InnoDB在不同隔离级别下的一致性读及锁的差异 ###

前面讲过，锁和多版本数据是InnoDB实现一致性读和ISO/ANSI SQL92隔离级别的手段，因此，在不同的隔离级别下，InnoDB处理SQL时采用的一致性读策略和需要的锁是不同的。同时，数据恢复和复制机制的特点，也对一些SQL的一致性读策略和锁策略有很大影响。将这些特性归纳成如表20-16所示的内容，以便读者查阅。

表20-16                                          InnoDB存储引擎中不同SQL在不同隔离级别下锁比较

<table style="border-collapse: collapse; border-style: none;"> 
 <tbody> 
 <tr style="height: 68.4pt;"> 
 <td style="padding-bottom: 0cm; padding-left: 5.4pt; width: 172.05pt; padding-right: 5.4pt; height: 68.4pt; padding-top: 0cm; border: windowtext 1pt solid;" colspan="2"> 隔离级别 &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 一致性读和锁 SQL </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; height: 68.4pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> Read Uncommited </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; height: 68.4pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> Read Commited </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; height: 68.4pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> Repeatable Read </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; height: 68.4pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> Serializable </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: 85.5pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> SQL </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 条件 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; 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: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; 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: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; 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: 66.95pt; 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: 85.5pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> select </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 相等 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> None locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Consisten read/None lock </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Consisten read/None lock </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share locks </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 范围 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> None locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Consisten read/None lock </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Consisten read/None lock </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </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: 85.5pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> update </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 相等 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Exclusive locks </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 范围 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </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: 85.5pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> Insert </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> N/A </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </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: 85.5pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> replace </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 无键冲突 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 键冲突 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </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: 85.5pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> delete </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 相等 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 范围 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </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: 85.5pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> Select ... from ... Lock in share mode </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 相等 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share locks </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 范围 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </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: 85.5pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> Select * from ... For update </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 相等 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 范围 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> exclusive next-key </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: 85.5pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> Insert into ... Select ... （指源表锁） </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> innodb_locks_unsafe_for_binlog=off </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> innodb_locks_unsafe_for_binlog=on </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> None locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Consisten read/None lock </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Consisten read/None lock </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </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: 85.5pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> create table ... Select ... （指源表锁） </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> innodb_locks_unsafe_for_binlog=off </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </td> 
 </tr> 
 <tr> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 86.55pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> innodb_locks_unsafe_for_binlog=on </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.45pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> None locks </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 65.5pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Consisten read/None lock </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.1pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Consisten read/None lock </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 66.95pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Share Next-Key </td> 
 </tr> 
 </tbody> 
</table>

从表20-16可以看出：对于许多SQL，隔离级别越高，InnoDB给记录集加的锁就越严格（尤其是使用范围条件的时候），产生锁冲突的可能性也就越高，从而对并发性事务处理性能的影响也就越大。因此，我们在应用中，应该尽量使用较低的隔离级别，以减少锁争用的机率。实际上，通过优化事务逻辑，大部分应用使用Read Commited隔离级别就足够了。对于一些确实需要更高隔离级别的事务，可以通过在程序中执行SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ或SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE动态改变隔离级别的方式满足需求。

### 什么时候使用表锁 ###

对于InnoDB表，在绝大部分情况下都应该使用行级锁，因为事务和行锁往往是我们之所以选择InnoDB表的理由。但在个别特殊事务中，也可以考虑使用表级锁。

¡  第一种情况是：事务需要更新大部分或全部数据，表又比较大，如果使用默认的行锁，不仅这个事务执行效率低，而且可能造成其他事务长时间锁等待和锁冲突，这种情况下可以考虑使用表锁来提高该事务的执行速度。

¡  第二种情况是：事务涉及多个表，比较复杂，很可能引起死锁，造成大量事务回滚。这种情况也可以考虑一次性锁定事务涉及的表，从而避免死锁、减少数据库因事务回滚带来的开销。

当然，应用中这两种事务不能太多，否则，就应该考虑使用MyISAM表了。

在InnoDB下，使用表锁要注意以下两点。

（1）使用LOCK TABLES虽然可以给InnoDB加表级锁，但必须说明的是，表锁不是由InnoDB存储引擎层管理的，而是由其上一层──MySQL Server负责的，仅当autocommit=0、innodb\_table\_locks=1（默认设置）时，InnoDB层才能知道MySQL加的表锁，MySQL Server也才能感知InnoDB加的行锁，这种情况下，InnoDB才能自动识别涉及表级锁的死锁；否则，InnoDB将无法自动检测并处理这种死锁。有关死锁，下一小节还会继续讨论。

（2）在用LOCK TABLES对InnoDB表加锁时要注意，要将AUTOCOMMIT设为0，否则MySQL不会给表加锁；事务结束前，不要用UNLOCK TABLES释放表锁，因为UNLOCK TABLES会隐含地提交事务；COMMIT或ROLLBACK并不能释放用LOCK TABLES加的表级锁，必须用UNLOCK TABLES释放表锁。正确的方式见如下语句：

例如，如果需要写表t1并从表t读，可以按如下做：

SET AUTOCOMMIT=0;

LOCK TABLES t1 WRITE, t2 READ, ...;

\[do something with tables t1 and t2 here\];

COMMIT;

UNLOCK TABLES;

### 关于死锁 ###

上文讲过，MyISAM表锁是deadlock free的，这是因为MyISAM总是一次获得所需的全部锁，要么全部满足，要么等待，因此不会出现死锁。但在InnoDB中，除单个SQL组成的事务外，锁是逐步获得的，这就决定了在InnoDB中发生死锁是可能的。如表20-17所示的就是一个发生死锁的例子。

表20-17                   InnoDB存储引擎中的死锁例子

<table style="border-collapse: collapse; margin-left: 10.1pt; border-style: none;"> 
 <tbody> 
 <tr style="height: 31.3pt;"> 
 <td style="width: 181.35pt; height: 31.3pt; border: windowtext 1pt solid; padding: 0cm;"> session_1 </td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; height: 31.3pt; border-left-color: windowtext; border-top: windowtext 1pt solid; border-left-width: 1pt; border-right: windowtext 1pt solid; padding: 0cm;"> session_2 </td> 
 </tr> 
 <tr style="page-break-inside: avoid; height: 2cm;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 181.35pt; border-top-width: 1pt; height: 2cm; border-right: windowtext 1pt solid; padding: 0cm;"> mysql&gt; set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql&gt; select * from table_1 where where id=1 for update; ... 做一些其他处理... </td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; height: 2cm; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;"> mysql&gt; set autocommit = 0; Query OK, 0 rows affected (0.00 sec) mysql&gt; select * from table_2 where id=1 for update; ... </td> 
 </tr> 
 <tr style="page-break-inside: avoid;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 181.35pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding: 0cm;"> select * from table_2 where id =1 for update; 因session_2已取得排他锁，等待 </td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;"> 做一些其他处理... </td> 
 </tr> 
 <tr style="page-break-inside: avoid; height: 41.75pt;"> 
 <td style="border-bottom: windowtext 1pt solid; border-left: windowtext 1pt solid; border-top-color: windowtext; width: 181.35pt; border-top-width: 1pt; height: 41.75pt; border-right: windowtext 1pt solid; padding: 0cm;">&nbsp;</td> 
 <td style="border-bottom: windowtext 1pt solid; width: 202.5pt; border-top-style: none; height: 41.75pt; border-left-style: none; border-right: windowtext 1pt solid; padding: 0cm;"> mysql&gt; select * from table_1 where where id=1 for update; 死锁 </td> 
 </tr> 
 </tbody> 
</table>

在上面的例子中，两个事务都需要获得对方持有的排他锁才能继续完成事务，这种循环锁等待就是典型的死锁。

发生死锁后，InnoDB一般都能自动检测到，并使一个事务释放锁并回退，另一个事务获得锁，继续完成事务。但在涉及外部锁，或涉及表锁的情况下，InnoDB并不能完全自动检测到死锁，这需要通过设置锁等待超时参数innodb\_lock\_wait\_timeout来解决。需要说明的是，这个参数并不是只用来解决死锁问题，在并发访问比较高的情况下，如果大量事务因无法立即获得所需的锁而挂起，会占用大量计算机资源，造成严重性能问题，甚至拖跨数据库。我们通过设置合适的锁等待超时阈值，可以避免这种情况发生。

通常来说，死锁都是应用设计的问题，通过调整业务流程、数据库对象设计、事务大小，以及访问数据库的SQL语句，绝大部分死锁都可以避免。下面就通过实例来介绍几种避免死锁的常用方法。

（1）在应用中，如果不同的程序会并发存取多个表，应尽量约定以相同的顺序来访问表，这样可以大大降低产生死锁的机会。在下面的例子中，由于两个session访问两个表的顺序不同，发生死锁的机会就非常高！但如果以相同的顺序来访问，死锁就可以避免。

表20-18        InnoDB存储引擎中表顺序造成的死锁例子

<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: 19.45pt;"> 
 <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: 19.45pt; 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: 207pt; padding-right: 5.4pt; border-top-style: none; height: 19.45pt; 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: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; select first_name,last_name from actor where actor_id = 1 for update; +------------+-----------+ | first_name | last_name | +------------+-----------+ | PENELOPE&nbsp;&nbsp; | GUINESS&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;"> mysql&gt; insert into country (country_id,country) values(110,'Test'); Query OK, 1 row 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: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt;&nbsp; insert into country (country_id,country) values(110,'Test'); 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;"> mysql&gt; select first_name,last_name from actor where actor_id = 1 for update; +------------+-----------+ | first_name | last_name | +------------+-----------+ | PENELOPE&nbsp;&nbsp; | GUINESS&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;"> mysql&gt;&nbsp; insert into country (country_id,country) values(110,'Test'); ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction </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>

（2）在程序以批量方式处理数据的时候，如果事先对数据排序，保证每个线程按固定的顺序来处理记录，也可以大大降低出现死锁的可能。

表20-19        InnoDB存储引擎中表数据操作顺序不一致造成的死锁例子

<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: 19.45pt;"> 
 <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: 19.45pt; 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: 207pt; padding-right: 5.4pt; border-top-style: none; height: 19.45pt; 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: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; select first_name,last_name from actor where actor_id = 1 for update; +------------+-----------+ | first_name | last_name | +------------+-----------+ | PENELOPE&nbsp;&nbsp; | GUINESS&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;"> mysql&gt; select first_name,last_name from actor where actor_id = 3 for update; +------------+-----------+ | first_name | last_name | +------------+-----------+ | ED&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | CHASE&nbsp;&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;"> mysql&gt; select first_name,last_name from actor where actor_id = 3 for update; 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;"> mysql&gt; select first_name,last_name from actor where actor_id = 1 for update; 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;"> mysql&gt; select first_name,last_name from actor where actor_id = 3 for update; +------------+-----------+ | first_name | last_name | +------------+-----------+ | ED&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | CHASE&nbsp;&nbsp;&nbsp;&nbsp; | +------------+-----------+ 1 row in set (4.71 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> 
 </tbody> 
</table>

（3）在事务中，如果要更新记录，应该直接申请足够级别的锁，即排他锁，而不应先申请共享锁，更新时再申请排他锁，因为当用户申请排他锁时，其他事务可能又已经获得了相同记录的共享锁，从而造成锁冲突，甚至死锁。具体演示可参见20.3.3小节中的例子。

（4）前面讲过，在REPEATABLE-READ隔离级别下，如果两个线程同时对相同条件记录用SELECT...FOR UPDATE加排他锁，在没有符合该条件记录情况下，两个线程都会加锁成功。程序发现记录尚不存在，就试图插入一条新记录，如果两个线程都这么做，就会出现死锁。这种情况下，将隔离级别改成READ COMMITTED，就可避免问题，如表20-20所示。

表20-20                     InnoDB存储引擎中隔离级别引起的死锁例子1

<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; select @@tx_isolation; +-----------------+ | @@tx_isolation&nbsp; | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) 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: 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; select @@tx_isolation; +-----------------+ | @@tx_isolation&nbsp; | +-----------------+ | REPEATABLE-READ | +-----------------+ 1 row in set (0.00 sec) 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: 216pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> 当前session对不存在的记录加for update的锁： mysql&gt; select actor_id,first_name,last_name from actor where actor_id = 201 for update; Empty 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也可以对不存在的记录加for update的锁： mysql&gt; select actor_id,first_name,last_name from actor where actor_id = 201 for update; Empty 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;"> 因为其他session也对该记录加了锁，所以当前的插入会等待： mysql&gt; insert into actor (actor_id , first_name , last_name) values(201,'Lisa','Tom'); 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;"> 因为其他session已经对记录进行了更新，这时候再插入记录就会提示死锁并退出： mysql&gt; insert into actor (actor_id, first_name , last_name) values(201,'Lisa','Tom'); 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;"> 由于其他session已经退出，当前session可以获得锁并成功插入记录： mysql&gt; insert into actor (actor_id , first_name , last_name) values(201,'Lisa','Tom'); Query OK, 1 row affected (13.35 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> 
 </tbody> 
</table>

（5）当隔离级别为READ COMMITTED时，如果两个线程都先执行SELECT...FOR UPDATE，判断是否存在符合条件的记录，如果没有，就插入记录。此时，只有一个线程能插入成功，另一个线程会出现锁等待，当第1个线程提交后，第2个线程会因主键重出错，但虽然这个线程出错了，却会获得一个排他锁！这时如果有第3个线程又来申请排他锁，也会出现死锁。

对于这种情况，可以直接做插入操作，然后再捕获主键重异常，或者在遇到主键重错误时，总是执行ROLLBACK释放获得的排他锁，如表20-21所示。

表20-21                        InnoDB存储引擎中隔离级别引起的死锁例子2

<table style="border-collapse: collapse; margin-left: -3.6pt; border-style: none;"> 
 <tbody> 
 <tr> 
 <td style="padding-bottom: 0cm; padding-left: 5.4pt; width: 144pt; 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: 144pt; 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> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 144pt; 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_3 </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: 144pt; padding-right: 5.4pt; border-top-width: 1pt; height: 2cm; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; select @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | READ-COMMITTED | +----------------+ 1 row in set (0.00 sec) mysql&gt; set autocommit=0; Query OK, 0 rows affected (0.01 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 144pt; padding-right: 5.4pt; border-top-style: none; height: 2cm; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; select @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | READ-COMMITTED | +----------------+ 1 row in set (0.00 sec) mysql&gt; set autocommit=0; Query OK, 0 rows affected (0.01 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 144pt; padding-right: 5.4pt; border-top-style: none; height: 2cm; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> mysql&gt; select @@tx_isolation; +----------------+ | @@tx_isolation | +----------------+ | READ-COMMITTED | +----------------+ 1 row in set (0.00 sec) mysql&gt; set autocommit=0; Query OK, 0 rows affected (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: 144pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> Session_1获得for update的共享锁： mysql&gt; select actor_id, first_name,last_name from actor where actor_id = 201 for update; Empty set (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 144pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 由于记录不存在，session_2也可以获得for update的共享锁： mysql&gt; select actor_id, first_name,last_name from actor where actor_id = 201 for update; Empty set (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 144pt; 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: 144pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> Session_1可以成功插入记录： mysql&gt; insert into actor (actor_id,first_name,last_name) values(201,'Lisa','Tom'); Query OK, 1 row affected (0.00 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 144pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; 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: 144pt; 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: 144pt; 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: 144pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Session_2插入申请等待获得锁： mysql&gt; insert into actor (actor_id,first_name,last_name) values(201,'Lisa','Tom'); 等待 </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 144pt; 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: 144pt; padding-right: 5.4pt; border-top-width: 1pt; border-right: windowtext 1pt solid; padding-top: 0cm;"> Session_1成功提交： mysql&gt; commit; Query OK, 0 rows affected (0.04 sec) </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 144pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; 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: 144pt; 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: 144pt; 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: 144pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Session_2获得锁，发现插入记录主键重，这个时候抛出了异常，但是并没有释放共享锁： mysql&gt; insert into actor (actor_id,first_name,last_name) values(201,'Lisa','Tom'); ERROR 1062 (23000): Duplicate entry '201' for key 'PRIMARY' </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 144pt; 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: 144pt; 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: 144pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; 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: 144pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Session_3申请获得共享锁，因为session_2已经锁定该记录，所以session_3需要等待： mysql&gt; select actor_id, first_name,last_name from actor where actor_id = 201 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: 144pt; 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: 144pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> 这个时候，如果session_2直接对记录进行更新操作，则会抛出死锁的异常： mysql&gt; update actor set last_name='Lan' where actor_id = 201; ERROR 1213 (40001): Deadlock found when trying to get lock; try restarting transaction </td> 
 <td style="border-bottom: windowtext 1pt solid; padding-bottom: 0cm; padding-left: 5.4pt; width: 144pt; 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: 144pt; 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: 144pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; 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: 144pt; padding-right: 5.4pt; border-top-style: none; border-left-style: none; border-right: windowtext 1pt solid; padding-top: 0cm;"> Session_2释放锁后，session_3获得锁： mysql&gt; select first_name, last_name from actor where actor_id = 201 for update; +------------+-----------+ | first_name | last_name | +------------+-----------+ | Lisa&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | Tom&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; | +------------+-----------+ 1 row in set (31.12 sec) </td> 
 </tr> 
 </tbody> 
</table>

尽管通过上面介绍的设计和SQL优化等措施，可以大大减少死锁，但死锁很难完全避免。因此，在程序设计中总是捕获并处理死锁异常是一个很好的编程习惯。

如果出现死锁，可以用SHOW INNODB STATUS命令来确定最后一个死锁产生的原因。返回结果中包括死锁相关事务的详细信息，如引发死锁的SQL语句，事务已经获得的锁，正在等待什么锁，以及被回滚的事务等。据此可以分析死锁产生的原因和改进措施。下面是一段SHOW INNODB STATUS输出的样例：

mysql> show innodb status /G

…….

\------------------------

LATEST DETECTED DEADLOCK

\------------------------

070710 14:05:16

\*\*\* (1) TRANSACTION:

TRANSACTION 0 117470078, ACTIVE 117 sec, process no 1468, OS thread id 1197328736 inserting

mysql tables in use 1, locked 1

LOCK WAIT 5 lock struct(s), heap size 1216

MySQL thread id 7521657, query id 673468054 localhost root update

insert into country (country\_id,country) values(110,'Test')