Mysql加锁过程详解--03关于mysql幻读理解

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深入探讨MySQL InnoDB如何通过next-key locks机制避免幻读现象，解释在可重复读隔离级别下，如何使用加锁读来确保数据的一致性和避免幻读。

转载自：http://www.cnblogs.com/crazylqy/p/7614092.html

Mysql加锁过程详解（2）-关于mysql 幻读理解出现了幻读，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

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。

下面看列子

例子1

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

例子2

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

例子3

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

例子 4

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

例子 5

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，forupdate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）

------------------------------------------------------------------

再来看大神的解释：链接: 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     |
| +----+-------+
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

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