mysql的可重复读REPEATABLE READ隔离级别和幻读

1）mvcc多版本控制提高读写qps

2) REPEATBLE READ 级别并不能完全避免幻读，需要加next key locks，可以使显示锁(select * where * for update or lock in share mode)

一些文章写到InnoDB的可重复读避免了“幻读”（phantom read），这个说法并不准确。

做个试验：(以下所有试验要注意存储引擎和隔离级别)

mysql>show create table t_bitfly/G;

CREATE TABLE `t_bitfly` (

`id` bigint(20) NOT NULL default '0',

`value` varchar(32) default NULL,

PRIMARY KEY (`id`)

) ENGINE=InnoDB DEFAULT CHARSET=gbk

mysql>select @@global.tx_isolation, @@tx_isolation;

+-----------------------+-----------------+

| @@global.tx_isolation | @@tx_isolation |

+-----------------------+-----------------+

| REPEATABLE-READ | REPEATABLE-READ |

+-----------------------+-----------------+

试验4-1：

tSessionA Session B

|

| START TRANSACTION; START TRANSACTION;

|

| SELECT * FROM t_bitfly;

| empty set

| INSERT INTO t_bitfly VALUES (1, 'a');

|

|

| SELECT * FROM t_bitfly;

| empty set

| COMMIT;

|

| SELECT * FROM t_bitfly;

| empty set

|

| INSERT INTO t_bitfly VALUES (1, 'a');

| ERROR 1062 (23000):

| Duplicate entry '1' for key 1

v (shit,刚刚明明告诉我没有这条记录的)

如此就出现了幻读，以为表里没有数据，其实数据已经存在了，傻乎乎的提交后，才发现数据冲突了。

试验4-2：

tSessionA Session B

|

| START TRANSACTION; START TRANSACTION;

|

| SELECT * FROM t_bitfly;

| +------+-------+

| | id | value |

| +------+-------+

| | 1 |a |

| +------+-------+

| INSERT INTO t_bitfly VALUES (2, 'b');

|

|

| SELECT * FROM t_bitfly;

| +------+-------+

| | id | value |

| +------+-------+

| | 1 |a |

| +------+-------+

| COMMIT;

|

| SELECT * FROM t_bitfly;

| +------+-------+

| | id | value |

| +------+-------+

| | 1 |a |

| +------+-------+

|

| UPDATE t_bitfly SET value='z';

| Rows matched: 2 Changed:2 Warnings: 0

| (怎么多出来一行)

|

| SELECT * FROM t_bitfly;

| +------+-------+

| | id | value |

| +------+-------+

| | 1 |z |

| | 2 |z |

| +------+-------+

|

v

本事务中第一次读取出一行，做了一次更新后，另一个事务里提交的数据就出现了。也可以看做是一种幻读。

------

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
By default, InnoDB operatesin REPEATABLE READ transaction isolation level and with the innodb_locks_unsafe_for_binlogsystem variable disabled. In this case, InnoDB uses next-key locks for searchesand index scans, which prevents phantom rows (see Section 13.6.8.5,
“Avoidingthe 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-keylocks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 PhantomProblem Using Next-Key Locking (http://dev.mysql.com/doc/refman/5.0/en/innodb-next-key-locking.html)

Toprevent phantoms,

InnoDB

usesan algorithm called next-key locking that combinesindex-row locking with gap locking.

Youcan 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 youare going to insert, then you can safely insert your row and know that thenext-key lock set on the successor
of your row during the read prevents anyonemeanwhile inserting a duplicate for your row. Thus, the next-key lockingenables 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showinnodb status来查看是否给表加上了锁。

再看一个实验，要注意，表t_bitfly里的id为主键字段。

实验4-3：

t SessionA 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
Forlocking reads (

SELECT

with

FORUPDATE

or

LOCK IN SHARE MODE

),

UPDATE

, and

DELETE

statements,
lockingdepends on whether the statement uses a unique index with a unique searchcondition, or a range-type search condition. For a unique index with a uniquesearch condition,

InnoDB

locksonly the index record found,
not the gap before it. For other searchconditions,

InnoDB

locksthe 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.

------

一致性读和提交读，先看实验，

实验4-4：

tSessionA Session B

|

| STARTTRANSACTION; 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
Ifyou want to see the “freshest” state of the database, you should use either theREAD COMMITTED isolation level or a locking read:

SELECT * FROM t_bitfly LOCK IN SHARE MODE;

------

结论：MySQLInnoDB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-keylocks。