数据库基础之锁(2. 幻读理解)
转载自Mysql加锁过程详解(2)-关于mysql 幻读理解,系列都很推荐!在此仅作备份与温故!
正文
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?
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; |
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SET AUTOCOMMIT=0; |
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1.不可重复读 |
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begin |
begin |
INSERT test VALUES(1,1); |
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SELECT * FROM test;
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SELECT * FROM test;
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commit |
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SELECT * FROM test; |
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COMMIT |
B在一个事务的查询的结果变了,不可重复读 |
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2.锁 |
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begin |
begin |
INSERT test VALUES(2,2); |
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SELECT * FROM test;
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INSERT test VALUES(2,2); |
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Lock wait timeout exceeded; try restarting transaction |
COMMIT |
COMMIT |
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begin |
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INSERT test VALUES(3,3); |
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INSERT test VALUES(4,4); |
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COMMIT |
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BEGIN |
BEGIN |
SELECT COUNT(*) FROM test WHERE a>2;
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SELECT COUNT(*) FROM test WHERE a>2;
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INSERT test VALUES(5,5); |
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SELECT COUNT(*) FROM test WHERE a>2;
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SELECT COUNT(*) FROM test WHERE a>2;
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COMMIT |
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SELECT COUNT(*) FROM test WHERE a>2; |
SELECT COUNT(*) FROM test WHERE a>2; |
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例二:重复读
a |
b |
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; |
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SET AUTOCOMMIT=0; |
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1.可重复读 |
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begin |
begin |
INSERT test VALUES(1,1); |
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SELECT * FROM test;
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SELECT * FROM test;
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commit |
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SELECT * FROM test; |
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COMMIT |
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BEGIN |
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SELECT * FROM test; |
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COMMIT |
B在一个事务的查询的没变 |
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2锁 |
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begin |
begin |
INSERT test VALUES(2,2); |
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SELECT * FROM test;
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INSERT test VALUES(2,2); |
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Lock wait timeout exceeded; try restarting transaction |
COMMIT |
COMMIT |
3(幻读) |
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BEGIN |
BEGIN |
INSERT test VALUES(3,3); |
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SELECT * FROM test;
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SELECT * FROM test;
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COMMIT |
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SELECT * FROM test;
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INSERT test VALUES(3,3); |
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Duplicate entry '3' for key 'PRIMARY' |
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COMMIT |
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BEGIN |
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SELECT * FROM test;
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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? |
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begin |
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INSERT test VALUES(4,4); |
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COMMIT |
4.可重复读 |
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BEGIN |
BEGIN |
SELECT COUNT(*) FROM test WHERE a>2;
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SELECT COUNT(*) FROM test WHERE a>2;
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INSERT test VALUES(5,5); |
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SELECT COUNT(*) FROM test WHERE a>2;
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SELECT COUNT(*) FROM test WHERE a>2;
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COMMIT |
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BEGIN |
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SELECT COUNT(*) FROM test WHERE a>2; |
SELECT COUNT(*) FROM test WHERE a>2; |
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COMMIT |
COMMIT |
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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; |
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SET AUTOCOMMIT=0; |
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BEGIN |
BEGIN |
SELECT * FROM test WHERE a='1' FOR UPDATE; |
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SELECT * FROM test |
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INSERT test VALUES(1,1); |
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锁住了 |
INSERT test VALUES(1,1); |
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成功 |
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COMMIT |
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COMMIT |
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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; |
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SET AUTOCOMMIT=0; |
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BEGIN |
BEGIN |
SELECT * FROM test WHERE a='1' FOR UPDATE; |
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SELECT * FROM test |
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INSERT test VALUES(2,2); |
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连2也被锁住了? |
INSERT test VALUES(1,1); |
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成功 |
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COMMIT |
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这次提交成功 |
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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; |
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SET AUTOCOMMIT=0; |
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BEGIN |
BEGIN |
SELECT * FROM test |
SELECT * FROM test |
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SELECT * FROM test WHERE a='1' FOR UPDATE; |
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SELECT * FROM test |
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INSERT test VALUES(2,2); |
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COMMIT |
COMMIT |
成功 |
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COMMIT |
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COMMIT |
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例四:避免幻读(for update 锁住)
a |
b |
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; |
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SET AUTOCOMMIT=0; |
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BEGIN |
BEGIN |
SELECT * FROM test |
SELECT * FROM test |
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SELECT * FROM test WHERE a='2' FOR UPDATE; |
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SELECT * FROM test |
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INSERT test VALUES(2,2); |
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INSERT test VALUES(5,5); |
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COMMIT |
COMMIT |
例五:幻读与锁
a |
b |
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ; |
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SET AUTOCOMMIT=0; |
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BEGIN |
BEGIN |
SELECT * FROM test |
SELECT * FROM test |
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SELECT * FROM test WHERE a='1' FOR UPDATE; |
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INSERT test VALUES(5,5); |
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插入5成功了 |
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UPDATE test SET b=33 WHERE a='3' |
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INSERT test VALUES(2,2); |
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2也可以 |
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UPDATE test SET b=11 WHERE a='1' |
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1锁住了 |
COMMIT |
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COMMIT |
SELECT * FROM test |
SELECT * FROM test |
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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
withFOR UPDATE
orLOCK IN SHARE MODE
),UPDATE
, andDELETE
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