出现幻读
首先，mysql 幻读并非是”一个事务内进行两次相同操作居然得到了不一样的结果”，因为它根本不可能发生在使用了 read view / MVCC 的 RR 隔离级别下，这种幻读的定义更适合给 Oracle，Oracle 的事务隔离只有两级，RC 和 Serializable。然后还有很多人辩解说不可重复读是针对某条记录的，幻读是针对记录集合的，这是在自我安慰么？
这里给出 mysql 幻读的比较形象的场景：
users： id 主键
- T1：select * from users where id = 1;
- T2：insert into `users`(`id`, `name`) values (1, 'big cat');
- T1：insert into `users`(`id`, `name`) values (1, 'big cat');
- T1 ：主事务，检测表中是否有id为1的记录，没有则插入，这是我们期望的正常业务逻辑。
- T2 ：干扰事务，目的在于扰乱T1的正常的事务执行。
在 RR 隔离级别下，1、2是会正常执行的，3则会报错主键冲突，对于T1的业务来说是执行失败的，这里T1就是发生了幻读，因为T1读取的数据状态并不能支持他的下一步的业务，见鬼了一样。
在 Serializable 隔离级别下，1 执行时是会隐式的添加gap共享锁的，从而2会被阻塞，3会正常执行，对于T1来说业务是正确的，成功的扼杀了扰乱业务的T2，对于T1来说他读取的状态是可以拿来支持业务的。
所以mysql的幻读并非什么读取两次返回结果集不同，而是事务在插入事先检测不存在的记录时，惊奇地发现这些数据已经存在了，之前的检测读获取到的数据如同鬼影一般。
这里要灵活的理解读取的意思，第一次select是读取，第二次的insert其实也属于隐式的读取，只不过是在mysql的机制中读取的，插入数据也是要先读取一下有没有主键冲突才能决定是否执行插入。
不可重复读侧重表达读-读，幻读则是说读-写，用写来证实读的是鬼影。
案例
SELECT VERSION();
例一；读提交
a | b |
SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED; | |
SET AUTOCOMMIT=0; | |
1.不可重复读 | |
begin | begin |
INSERT test VALUES(1,1); |
|
SELECT * FROM test; | SELECT * FROM test;
|
commit |
|
| SELECT * FROM test; |
|
|
| COMMIT |
B在一个事务的查询的结果变了，不可重复读 | |
2.锁 | |
begin | begin |
INSERT test VALUES(2,2); |
|
| SELECT * FROM test;
|
| INSERT test VALUES(2,2); |
| Lock wait timeout exceeded; try restarting transaction |
COMMIT | COMMIT |
| |
| begin |
| INSERT test VALUES(3,3); |
| INSERT test VALUES(4,4); |
| COMMIT |
|
|
BEGIN | BEGIN |
SELECT COUNT(*) FROM test WHERE a>2;
| SELECT COUNT(*) FROM test WHERE a>2;
|
|
|
INSERT test VALUES(5,5); |
|
SELECT COUNT(*) FROM test WHERE a>2;
| SELECT COUNT(*) FROM test WHERE a>2;
|
|
|
COMMIT |
|
SELECT COUNT(*) FROM test WHERE a>2; | SELECT COUNT(*) FROM test WHERE a>2; |
|
|
| |
例二：重复读
a | b |
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; | |
SET AUTOCOMMIT=0; | |
1．可重复读 | |
begin | begin |
INSERT test VALUES(1,1); |
|
SELECT * FROM test;
| SELECT * FROM test;
|
commit |
|
| SELECT * FROM test; |
|
|
| COMMIT |
| BEGIN |
| SELECT * FROM test; |
|
|
| COMMIT |
B在一个事务的查询的没变 | |
2锁 | |
begin | begin |
INSERT test VALUES(2,2); |
|
| SELECT * FROM test;
|
| INSERT test VALUES(2,2); |
| Lock wait timeout exceeded; try restarting transaction |
COMMIT | COMMIT |
3(幻读) | |
BEGIN | BEGIN |
INSERT test VALUES(3,3); |
|
SELECT * FROM test;
| SELECT * FROM test;
|
COMMIT |
|
| SELECT * FROM test;
|
| INSERT test VALUES(3,3); |
| Duplicate entry '3' for key 'PRIMARY' |
| COMMIT |
| BEGIN |
| SELECT * FROM test;
|
| COMMIT |
幻读，b明明查到没有，插入时候提示主键冲突，刚刚查询没有，出现幻觉？ | |
| |
|
|
| begin |
| INSERT test VALUES(4,4); |
| COMMIT |
4．可重复读 | |
BEGIN | BEGIN |
SELECT COUNT(*) FROM test WHERE a>2;
| SELECT COUNT(*) FROM test WHERE a>2;
|
|
|
INSERT test VALUES(5,5); |
|
SELECT COUNT(*) FROM test WHERE a>2;
| SELECT COUNT(*) FROM test WHERE a>2;
|
|
|
COMMIT |
|
BEGIN |
|
SELECT COUNT(*) FROM test WHERE a>2; | SELECT COUNT(*) FROM test WHERE a>2; |
|
|
COMMIT | COMMIT |
| |
网上很多说范围啊，count等等都是不对的，不用于幻读。
解决幻读
InnoDB指出的可以避免幻读：
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,
InnoDB uses 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。
案例
例一：幻读
a | b |
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; | |
SET AUTOCOMMIT=0; | |
|
|
BEGIN | BEGIN |
SELECT * FROM test WHERE a='1' FOR UPDATE; |
|
| SELECT * FROM test |
|
|
| INSERT test VALUES(1,1); |
|
锁住了 |
INSERT test VALUES(1,1); |
|
成功 |
|
COMMIT |
|
|
|
| COMMIT |
| 避免幻读可以select锁住，再insert |
例二：未命中，间隙锁
a | b |
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; | |
SET AUTOCOMMIT=0; | |
|
|
BEGIN | BEGIN |
SELECT * FROM test WHERE a='1' FOR UPDATE; |
|
| SELECT * FROM test |
|
|
| INSERT test VALUES(2,2); |
|
连2也被锁住了？ |
INSERT test VALUES(1,1); |
|
成功 |
|
COMMIT |
|
|
这次提交成功 |
| COMMIT |
其他尝试，这种情况无论插入2还是5都被锁住等等 | |
例三：命中，记录锁
a | b |
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; | |
SET AUTOCOMMIT=0; | |
|
|
BEGIN | BEGIN |
SELECT * FROM test | SELECT * FROM test |
|
|
SELECT * FROM test WHERE a='1' FOR UPDATE; |
|
| SELECT * FROM test |
|
|
| INSERT test VALUES(2,2); |
|
|
COMMIT | COMMIT |
成功 |
|
COMMIT |
|
| COMMIT |
|
|
例四：避免幻读（for update 锁住）
a | b |
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; | |
SET AUTOCOMMIT=0; | |
|
|
BEGIN | BEGIN |
SELECT * FROM test | SELECT * FROM test |
|
|
SELECT * FROM test WHERE a='2' FOR UPDATE; |
|
|
|
| SELECT * FROM test |
|
|
| INSERT test VALUES(2,2); |
|
|
| INSERT test VALUES(5,5); |
|
|
COMMIT | COMMIT |
例五：幻读与锁
a | b |
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; | |
SET AUTOCOMMIT=0; | |
BEGIN | BEGIN |
SELECT * FROM test | SELECT * FROM test |
|
|
SELECT * FROM test WHERE a='1' FOR UPDATE; |
|
| INSERT test VALUES(5,5); |
|
插入5成功了 |
| UPDATE test SET b=33 WHERE a='3' |
|
|
| INSERT test VALUES(2,2); |
|
2也可以 |
| UPDATE test SET b=11 WHERE a='1' |
|
1锁住了 |
COMMIT |
|
|
|
| COMMIT |
SELECT * FROM test | SELECT * FROM test |
|
|
以上例子说明，for update时候，id为主键，RR策略时候，锁住了的条件符合的行，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）
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再来看大神的解释 ：链接: http://blog.bitfly.cn/post/mysql-innodb-phantom-read/
实验
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 |
| +------+-------+
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
For locking reads (SELECT with
FOR UPDATE or LOCK IN SHARE MODE),UPDATE, and DELETE statements, locking depends on whether the statement uses a unique index 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.
------
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 |
| +----+-------+
- 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；
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
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。
Min是清明的茗
