Mysql加锁过程详解(3)-关于mysql 幻读理解
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; |
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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 |
例子2
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 |
例子3
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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例子 4
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 |
例子 5
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,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
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 |
| +----+-------+
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