MySQL InnoDB事务的隔离级别有四级，默认是“可重复读”（REPEATABLE READ）。
四个级别逐渐增强，每个级别解决一个问题。
借鉴并改造了一个搞笑的比喻：
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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=gbkmysql> select @@global.tx_isolation, @@tx_isolation;
+-----------------------+-----------------+
| @@global.tx_isolation | @@tx_isolation|
+-----------------------+-----------------+
| REPEATABLE-READ| REPEATABLE-READ |
+-----------------------+-----------------+
试验一：
t Session A
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。
试验二：
t Session A
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: 2Changed: 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。
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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 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 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)
To prevent phantoms,
InnoDBuses an algorithm 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。
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):
|