mysql隔离级别

原文链接

MySQL InnoDB事务的隔离级别有四级，默认是“可重复读”（REPEATABLE READ）。

未提交读（READ UNCOMMITTED）。另一个事务修改了数据，但尚未提交，而本事务中的SELECT会读到这些未被提交的数据（脏读）。

提交读（READ COMMITTED）。本事务读取到的是最新的数据（其他事务提交后的）。问题是，在同一个事务里，前后两次相同的SELECT会读到不同的结果（不重复读）。

可重复读（REPEATABLE READ）。在同一个事务里，SELECT的结果是事务开始时时间点的状态，因此，同样的SELECT操作读到的结果会是一致的。但是，会有幻读现象（稍后解释）。

串行化（SERIALIZABLE）。读操作会隐式获取共享锁，可以保证不同事务间的互斥。

修改事务隔离级别的方法：

1.全局修改，修改my.cnf配置文件，在最后加上

#可选参数有：READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, SERIALIZABLE.
[mysqld]
transaction-isolation = REPEATABLE-READ

这里全局默认是REPEATABLE-READ,其实MySQL本来默认也是这个级别

2.对当前session修改，在登录mysql客户端后，执行命令：

set session transaction isolation level read uncommitted;

四个级别逐渐增强，每个级别解决一个问题。

脏读，最容易理解。另一个事务修改了数据，但尚未提交，而本事务中的SELECT会读到这些未被提交的数据。

不重复读。解决了脏读后，会遇到，同一个事务执行过程中，另外一个事务提交了新数据，因此本事务先后两次读到的数据结果会不一致。

幻读。解决了不重复读，保证了同一个事务里，查询的结果都是事务开始时的状态（一致性）。但是，如果另一个事务同时提交了新数据，本事务再更新时，就会“惊奇的”发现了这些新数据，貌似之前读到的数据是“鬼影”一样的幻觉。

借鉴并改造了一个搞笑的比喻：

脏读。假如，中午去食堂打饭吃，看到一个座位被同学小Q占上了，就认为这个座位被占去了，就转身去找其他的座位。不料，这个同学小Q起身走了。事实：该同学小Q只是临时坐了一小下，并未“提交”。

不重复读。假如，中午去食堂打饭吃，看到一个座位是空的，便屁颠屁颠的去打饭，回来后却发现这个座位却被同学小Q占去了。

幻读。假如，中午去食堂打饭吃，看到一个座位是空的，便屁颠屁颠的去打饭，回来后，发现这些座位都还是空的（重复读），窃喜。走到跟前刚准备坐下时，却惊现一个恐龙妹，严重影响食欲。仿佛之前看到的空座位是“幻影”一样。

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 actor\G;
*************************** 1. row ***************************
Table: actor
Create Table: CREATE TABLE `actor` (
`actor_id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`first_name` varchar(45) NOT NULL,
`last_name` varchar(45) NOT NULL,
`last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (`actor_id`),
KEY `idx_actor_last_name` (`last_name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

mysql> select @@global.tx_isolation, @@tx_isolation;
+-----------------------+-----------------+
| @@global.tx_isolation | @@tx_isolation  |
+-----------------------+-----------------+
| REPEATABLE-READ       | REPEATABLE-READ |
+-----------------------+-----------------+
1 row in set (0.00 sec)

试验一：

t Session A                   Session B
|
| START TRANSACTION;          START TRANSACTION;
|
| select * from actor;
| empty set
|                             insert into actor
|                             select * from sakila.actor limit 1;
|
| select * from actor;
| empty set
|                             COMMIT;
|
| select * from actor;
| empty set
|
| insert into actor select * from sakila.actor limit 1;
| ERROR 1062 (23000):
| Duplicate entry '1' for key 'PRIMARY'
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 actor_id from actor;
|   +----------+
|   | actor_id |
|   +----------+
|   |        1 |
|   +----------+
|                            insert into actor
|                            select * from sakila.actor where actor_id=2;
|
| select actor_id from actor;
|   +----------+
|   | actor_id |
|   +----------+
|   |        1 |
|   +----------+
|                            COMMIT;
|
| select actor_id from actor;
|   +----------+
|   | actor_id |
|   +----------+
|   |        1 |
|   +----------+
|
| update actor set last_update=now();
| Rows matched: 2  Changed: 2  Warnings: 0
| (怎么多出来一行)
|
| select actor_id from actor;
|   +----------+
|   | actor_id |
|   +----------+
|   |        1 |
|   |        2 |
|   +----------+
|
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。

那么，InnoDB指出的可以避免幻读是怎么回事呢？

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。

再看一个实验，要注意，表actor里的actor_id为主键字段。实验三：

t Session A                 Session B
|
| START TRANSACTION;        START TRANSACTION;
|
| select * from actor
|  where actor_id <=3
| FOR UPDATE;
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
|        3 | t3         | t3        | 2016-06-16 15:18:33 |
+----------+------------+-----------+---------------------+
|                           insert into actor
|                           values (2,'t2','t2',now());
|                           ERROR 1205 (HY000): Lock wait timeout exceeded;
|                           try restarting transaction
|
| select * from actor;
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
|        3 | t3         | t3        | 2016-06-16 15:18:33 |
|        4 | t0         | t0        | 2016-06-16 15:18:36 |
+----------+------------+-----------+---------------------+
|                           insert into actor
|                           values (5,'t5','t5',now());
|                           Query OK, 1 row affected (0.00 sec)
|
| select * from actor;
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
|        3 | t3         | t3        | 2016-06-16 15:18:33 |
|        4 | t0         | t0        | 2016-06-16 15:18:36 |
+----------+------------+-----------+---------------------+
|                           COMMIT;
|
| select * from actor;
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
|        3 | t3         | t3        | 2016-06-16 15:18:33 |
|        4 | t0         | t0        | 2016-06-16 15:18:36 |
+----------+------------+-----------+---------------------+
v

可以看到，用actor_id <=3加的锁，只锁住了actor_id <=3的范围，可以成功添加actor_id 为5的记录，添加actor_id 为2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：

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 actor;
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
|        3 | t3         | t3        | 2016-06-16 15:18:33 |
|        4 | t0         | t0        | 2016-06-16 15:18:36 |
|        5 | t5         | t5        | 2016-06-16 16:03:51 |
+----------+------------+-----------+---------------------+
|                                insert into actor
|                                values (6,'t6','t6',now());
|                                COMMIT;
|
| select * from actor;
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
|        3 | t3         | t3        | 2016-06-16 15:18:33 |
|        4 | t0         | t0        | 2016-06-16 15:18:36 |
|        5 | t5         | t5        | 2016-06-16 16:03:51 |
+----------+------------+-----------+---------------------+
|
| select * from actor lock in share mode;
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
|        3 | t3         | t3        | 2016-06-16 15:18:33 |
|        4 | t0         | t0        | 2016-06-16 15:18:36 |
|        5 | t5         | t5        | 2016-06-16 16:03:51 |
|        6 | t6         | t6        | 2016-06-16 16:11:53 |
+----------+------------+-----------+---------------------+
|
| select * from actor for update;
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
|        3 | t3         | t3        | 2016-06-16 15:18:33 |
|        4 | t0         | t0        | 2016-06-16 15:18:36 |
|        5 | t5         | t5        | 2016-06-16 16:03:51 |
|        6 | t6         | t6        | 2016-06-16 16:11:53 |
+----------+------------+-----------+---------------------+
|
| select * from actor;
+----------+------------+-----------+---------------------+
| actor_id | first_name | last_name | last_update         |
+----------+------------+-----------+---------------------+
|        1 | PENELOPE   | GUINESS   | 2006-02-15 04:34:33 |
|        3 | t3         | t3        | 2016-06-16 15:18:33 |
|        4 | t0         | t0        | 2016-06-16 15:18:36 |
|        5 | t5         | t5        | 2016-06-16 16:03:51 |
+----------+------------+-----------+---------------------+
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。