Mysql加锁过程详解（3）-关于mysql 幻读理解

淩亂°似流年 2022-05-30 04:36 183阅读 0赞

[**Mysql加锁过程详解（2）-关于mysql 幻读理解**][Mysql_2_-_mysql]出现了幻读，那么不是说mysql的重复读解决了幻读的么？

那么，InnoDB指出的可以避免幻读是怎么回事呢？

> [http://dev.mysql.com/doc/refman/5.0/en/innodb-record-level-locks.html][http_dev.mysql.com_doc_refman_5.0_en_innodb-record-level-locks.html]
> 
> By default, InnoDB operates in REPEATABLE READ transaction isolation level and with the innodb\_locks\_unsafe\_for\_binlog system variable disabled. In this case, InnoDB uses next-key locks for searches and index scans, which prevents phantom(幽灵) rows (see Section 13.6.8.5, “Avoiding the Phantom Problem Using Next-Key Locking”).

准备的理解是，当隔离级别是可重复读，且禁用innodb\_locks\_unsafe\_for\_binlog的情况下，在搜索和扫描index的时候使用的next-key locks可以避免幻读。

关键点在于，是InnoDB默认对一个普通的查询也会加next-key locks，还是说需要应用自己来加锁呢？如果单看这一句，可能会以为InnoDB对普通的查询也加了锁，如果是，那和序列化（SERIALIZABLE）的区别又在哪里呢？

MySQL manual里还有一段：

> 13.2.8.5. Avoiding the Phantom Problem Using Next-Key Locking ([http://dev.mysql.com/doc/refman/5.0/en/innodb-next-key-locking.html][http_dev.mysql.com_doc_refman_5.0_en_innodb-next-key-locking.html])
> 
> To prevent phantoms, `InnoDB` uses an algorithm（算法） called next-key locking that combines index-row locking with gap locking.
> 
> You can use next-key locking to implement a uniqueness check in your application: If you read your data in share mode and do not see a duplicate for a row you are going to insert, then you can safely insert your row and know that the next-key lock set on the successor of your row during the read prevents anyone meanwhile inserting a duplicate for your row. Thus, the next-key locking enables you to “lock” the nonexistence of something in your table.

我的理解是说，InnoDB提供了next-key locks，但需要应用程序自己去加锁。manual里提供一个例子：

> SELECT \* FROM child WHERE id > 100 FOR UPDATE;

这样，InnoDB会给id大于100的行（假如child表里有一行id为102），以及100-102，102+的gap都加上锁。

可以使用show innodb status来查看是否给表加上了锁。

下面看列子

例子1

<table style="margin:0px;padding:0px;border:1px solid #C0C0C0;color:rgb(51,51,51);font-family:verdana, Arial, Helvetica, sans-serif;font-size:14px;background-color:rgb(255,255,255);width:878px;height:657px;"> 
 <tbody style="margin:0px;padding:0px;"> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;">a</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;">b</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td colspan="2" style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;">SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td colspan="2" style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;">SET AUTOCOMMIT=0;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">BEGIN</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">BEGIN</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test WHERE a='1' FOR UPDATE;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104010919-1938423655.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">INSERT test VALUES(1,1);</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;"><img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104015528-1725462896.png" alt="" style="padding:0px;border:0px;max-width:900px;">&nbsp;锁住了</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">INSERT test VALUES(1,1);</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">成功</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">COMMIT</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104020887-2039098789.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">COMMIT</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">避免幻读可以select锁住，再insert</td> 
 </tr> 
 </tbody> 
</table>

例子2

例子3

<table style="margin:0px;padding:0px;border:1px solid #C0C0C0;color:rgb(51,51,51);font-family:verdana, Arial, Helvetica, sans-serif;font-size:14px;background-color:rgb(255,255,255);width:880px;height:719px;"> 
 <tbody style="margin:0px;padding:0px;"> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">a</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">b</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td colspan="2" style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td colspan="2" style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SET AUTOCOMMIT=0;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">BEGIN</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">BEGIN</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104132481-1091031787.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104134481-12957166.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test WHERE a='1' FOR UPDATE;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104132481-1091031787.png" alt="" style="padding:0px;border:0px;max-width:900px;"></td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104138747-1332787466.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">INSERT test VALUES(2,2);</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104143903-1665145247.png" alt="" style="padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">COMMIT</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">COMMIT</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">成功</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">COMMIT</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">COMMIT</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 </tbody> 
</table>

（当存在值的时候，如果锁定某一个已存在的行记录，并且该查询条件是主键或者唯一键，则会只锁定改行记录？）

例子 4

<table style="margin:0px;padding:0px;border:1px solid #C0C0C0;color:rgb(51,51,51);font-family:verdana, Arial, Helvetica, sans-serif;font-size:14px;background-color:rgb(255,255,255);width:883px;height:637px;"> 
 <tbody style="margin:0px;padding:0px;"> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">a</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">b</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td colspan="2" style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td colspan="2" style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SET AUTOCOMMIT=0;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">BEGIN</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">BEGIN</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104304059-1851404658.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104305465-800294356.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test WHERE a='2' FOR UPDATE;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104309950-1610819395.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104314684-2021050414.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">INSERT test VALUES(2,2);</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104319825-1455889582.png" alt="" style="padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">INSERT test VALUES(5,5);</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104322059-2030975459.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">COMMIT</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">COMMIT</td> 
 </tr> 
 </tbody> 
</table>

锁表

例子 5

<table style="margin:0px;padding:0px;border:1px solid #C0C0C0;color:rgb(51,51,51);font-family:verdana, Arial, Helvetica, sans-serif;font-size:14px;background-color:rgb(255,255,255);width:880px;height:805px;"> 
 <tbody style="margin:0px;padding:0px;"> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">a</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">b</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td colspan="2" style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td colspan="2" style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SET AUTOCOMMIT=0;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">BEGIN</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">BEGIN</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104356481-1257224036.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104358012-352407420.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test WHERE a='1' FOR UPDATE;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">INSERT test VALUES(5,5);</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;"><img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104413856-2086703256.png" alt="" style="padding:0px;border:0px;max-width:900px;">&nbsp;插入5成功了</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">UPDATE test SET b=33 WHERE a='3'</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104419669-648847820.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">INSERT test VALUES(2,2);</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104426872-1225749677.png" alt="" style="padding:0px;border:0px;max-width:900px;">&nbsp;2也可以</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">UPDATE test SET b=11 WHERE a='1'</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;"><img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104438528-2085907744.png" alt="" style="padding:0px;border:0px;max-width:900px;">&nbsp;1锁住了</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">COMMIT</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104445669-1241399068.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">COMMIT</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">SELECT * FROM test</td> 
 </tr> 
 <tr style="margin:0px;padding:0px;"> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104454637-861544600.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 <td style="margin:0px;padding:3px;border-color:#C0C0C0;border-collapse:collapse;text-align:center;">&nbsp;<img src="https://images2017.cnblogs.com/blog/464291/201709/464291-20170930104500044-490420920.png" alt="" style="margin-bottom:0px;padding:0px;border:0px;max-width:900px;">&nbsp;</td> 
 </tr> 
 </tbody> 
</table>

以上例子说明，for update时候，id为主键（查询条件），RR策略时候，锁住了条件符合的行，但是如果条件找不到任何列，锁住的是整个表，（主键，唯一索引，非唯一索引，（insert，update对于gab锁不通），参考第一章，第七章,第九章）

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

再来看大神的解释 ：链接: [http://blog.bitfly.cn/post/mysql-innodb-phantom-read/][http_blog.bitfly.cn_post_mysql-innodb-phantom-read]

再看一个实验，要注意，表t\_bitfly里的id为主键字段。实验三：

> t Session A Session B  
> |  
> | START TRANSACTION; START TRANSACTION;  
> |  
> | SELECT \* FROM t\_bitfly  
> | WHERE id=1  
> | FOR UPDATE;  
> | +------+-------+  
> | | id | value |  
> | +------+-------+  
> | | 1 | a |  
> | +------+-------+  
> | INSERT INTO t\_bitfly  
> | VALUES (2, 'b');  
> | Query OK, 1 row affected  
> |  
> | SELECT \* FROM t\_bitfly;  
> | +------+-------+  
> | | id | value |  
> | +------+-------+  
> | | 1 | a |  
> | +------+-------+  
> | INSERT INTO t\_bitfly  
> | VALUES (0, '0');  
> | (waiting for lock ...  
> | then timeout)  
> | ERROR 1205 (HY000):  
> | Lock wait timeout exceeded;  
> | try restarting transaction  
> |  
> | SELECT \* FROM t\_bitfly;  
> | +------+-------+  
> | | id | value |  
> | +------+-------+  
> | | 1 | a |  
> | +------+-------+  
> | COMMIT;  
> |  
> | SELECT \* FROM t\_bitfly;  
> | +------+-------+  
> | | id | value |  
> | +------+-------+  
> | | 1 | a |  
> | +------+-------+  
> v

可以看到，用id<=1加的锁，只锁住了id<=1的范围，可以成功添加id为2的记录，添加id为0的记录时就会等待锁的释放。

MySQL manual（手册）里对可重复读里的锁的详细解释：

> [http://dev.mysql.com/doc/refman/5.0/en/set-transaction.html\#isolevel\_repeatable-read][http_dev.mysql.com_doc_refman_5.0_en_set-transaction.html_isolevel_repeatable-read]
> 
> For locking reads ([`SELECT`][SELECT] with `FOR UPDATE` or `LOCK IN SHARE MODE`),[`UPDATE`][UPDATE], and [`DELETE`][DELETE] statements, locking depends on whether the statement uses a unique index（唯一索引） with a unique search condition, or a range-type search condition. For a unique index with a unique search condition, `InnoDB` locks only the index record found, not the gap before it. For other search conditions, `InnoDB` locks the index range scanned, using gap locks or next-key (gap plus index-record) locks to block insertions by other sessions into the gaps covered by the range.

翻译：对于锁定读取（SELECT FOR WITH UPDATE或LOCK IN SHARE MODE），UPDATE和DELETE语句，锁定取决于语句是使用具有唯一搜索条件的唯一索引还是范围类型搜索条件。 对于具有唯一搜索条件的唯一索引，InnoDB只锁定找到的索引记录，而不是锁定之前的范围。 对于其他搜索条件，InnoDB使用间隙锁或next-key（间隙加索引记录）锁来锁定扫描的索引范围，以阻止其他会话插入到范围所覆盖的间隙中。

\------

一致性读和提交读，先看实验，实验四：

> t Session A Session B  
> |  
> | START TRANSACTION; START TRANSACTION;  
> |  
> | SELECT \* FROM t\_bitfly;  
> | +----+-------+  
> | | id | value |  
> | +----+-------+  
> | | 1 | a |  
> | +----+-------+  
> | INSERT INTO t\_bitfly  
> | VALUES (2, 'b');  
> | COMMIT;  
> |  
> | SELECT \* FROM t\_bitfly;  
> | +----+-------+  
> | | id | value |  
> | +----+-------+  
> | | 1 | a |  
> | +----+-------+  
> |  
> | SELECT \* FROM t\_bitfly LOCK IN SHARE MODE;  
> | +----+-------+  
> | | id | value |  
> | +----+-------+  
> | | 1 | a |  
> | | 2 | b |  
> | +----+-------+  
> |  
> | SELECT \* FROM t\_bitfly FOR UPDATE;  
> | +----+-------+  
> | | id | value |  
> | +----+-------+  
> | | 1 | a |  
> | | 2 | b |  
> | +----+-------+  
> |  
> | SELECT \* FROM t\_bitfly;  
> | +----+-------+  
> | | id | value |  
> | +----+-------+  
> | | 1 | a |  
> | +----+-------+  
> v

如果使用普通的读，会得到一致性的结果，如果使用了加锁的读，就会读到“最新的”“提交”读的结果。

本身，可重复读和提交读是矛盾的。在同一个事务里，如果保证了可重复读，就会看不到其他事务的提交，违背了提交读；如果保证了提交读，就会导致前后两次读到的结果不一致，违背了可重复读。

**可以这么讲，InnoDB提供了这样的机制，在默认的可重复读的隔离级别里，可以使用加锁读去查询最新的数据。**

> [http://dev.mysql.com/doc/refman/5.0/en/innodb-consistent-read.html][http_dev.mysql.com_doc_refman_5.0_en_innodb-consistent-read.html]
> 
> If you want to see the “freshest” state of the database, you should use either the READ COMMITTED isolation level or a locking read:  
> SELECT \* FROM t\_bitfly LOCK IN SHARE MODE;

结论：MySQL InnoDB的可重复读并不保证避免幻读，需要应用使用加锁读来保证。而这个加锁度使用到的机制就是next-key locks。

结论：mysql 的重复读解决了幻读的现象，但是需要 加上 select for update/lock in share mode 变成当前读避免幻读，普通读select存在幻读

在RR级别下普通读都是快照读而非当前读

原文地址：[http://www.cnblogs.com/crazylqy/p/7614092.html][http_www.cnblogs.com_crazylqy_p_7614092.html]

[Mysql_2_-_mysql]: http://www.cnblogs.com/crazylqy/p/7612230.html
[http_dev.mysql.com_doc_refman_5.0_en_innodb-record-level-locks.html]: http://dev.mysql.com/doc/refman/5.0/en/innodb-record-level-locks.html
[http_dev.mysql.com_doc_refman_5.0_en_innodb-next-key-locking.html]: http://dev.mysql.com/doc/refman/5.0/en/innodb-next-key-locking.html
[http_blog.bitfly.cn_post_mysql-innodb-phantom-read]: http://blog.bitfly.cn/post/mysql-innodb-phantom-read/
[http_dev.mysql.com_doc_refman_5.0_en_set-transaction.html_isolevel_repeatable-read]: http://dev.mysql.com/doc/refman/5.0/en/set-transaction.html#isolevel_repeatable-read
[SELECT]: http://dev.mysql.com/doc/refman/5.0/en/select.html
[UPDATE]: http://dev.mysql.com/doc/refman/5.0/en/update.html
[DELETE]: http://dev.mysql.com/doc/refman/5.0/en/delete.html
[http_dev.mysql.com_doc_refman_5.0_en_innodb-consistent-read.html]: http://dev.mysql.com/doc/refman/5.0/en/innodb-consistent-read.html
[http_www.cnblogs.com_crazylqy_p_7614092.html]: http://www.cnblogs.com/crazylqy/p/7614092.html