MySQL事务特性,隔离级别
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事务特性ACID
Atomic,原子：同一个事务里，要么都提交，要么都回滚；
Consistency,一致性：即在事务开始之前和事务结束以后，数据库的完整性约束没有被破坏；
Isolation,隔离：并发事务间的行数据是彼此隔离的；
Durability,持久：事务提交后，所有结果务必被持久化。
mysql支持事务的存储引擎：Innodb，NDBcluster，TokuD
MySQL不支持事务的存储引擎：myisam ，memory
1.隔离性通过锁的方式实现
2.原子性，一致性，持久性通过数据库的redo和undo来完成
撤销日志，undo log 没提交的事务撤销
重做日志，redo log 检查已经提交没有持久化的事务重做
显式事务启动|结束
1.以start transaction/begin开始事务
2.以commit/rollback transaction结束事务
隐形事务提交
主要是DDL,DCL会引发事务隐形提交 DDL语句 1.alter function 2.alter procedure 3.alter table 4.begin 5.create databases 6.create function 7.create index 8.create procedure 9.create table 10.drop databases 11.drop function 12.drop index 13.drop procedure 14.drop table 15.unlock tables 16.load master data 17.lock tables 18.rename table 19.truncate table 20.set autocommit=1 21.start transaction 22.create table…select 23.create temporary table ….select 除外 用户管理 1.create user 2.drop user 3.grant 4.rename user 5.revoke 6.set password 事务控制 1.begin 2.lock tables 3.set autocommit=1（if the valueis not already 1） 4.start transaction 5.unlock tables 6.lock tables unlock tables也会 7.flush tables with read lock & unlock table除外 数据导入 Load data infile 表管理语句 1.analyze table 2.cache index 3.check table 4.load index into cache 5.optimize table 6.repair table
事务隔离级别
Innodb采用next-key lock机制来避免幻读，RR+innodb_locks_unsafe_for_binlog=1，它的作用是事务隔离级别降为RC，只有record lock，没有gap lock。
set tx_isolation=\'read-uncommitted\'； select @@session.tx_isolation; +------------------------+ | @@session.tx_isolation | +------------------------+ | READ-UNCOMMITTED | +------------------------+ 1 row in set (0.00 sec)
脏读
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>begin; Query OK, 0 rows affected (0.00 sec) >select * from t5 where id=7; Empty set (0.00 sec) |
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>select * from t5 where id=7; Empty set (0.00 sec |
>insert into t5 select 7,\'wwb\',29,\'dba\',\'M\'; Query OK, 1 row affected (0.00 sec) Records: 1 Duplicates: 0 Warnings: 0 >select * from t5 where id=7; +----+------+------+---------+------+ | id | name | age | content | sex | +----+------+------+---------+------+ | 7 | wwb | 29 | dba | M | +----+------+------+---------+------+ 1 row in set (0.00 sec)
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>select * from t5 where id=7; +----+------+------+---------+------+ | id | name | age | content | sex | +----+------+------+---------+------+ | 7 | wwb | 29 | dba | M | +----+------+------+---------+------+ 1 row in set (0.00 sec)
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将事务隔离级别改为RC
>set tx_isolation=\'read-committed\'; QueryOK, 0 rows affected (0.00 sec) >select@@session.tx_isolation; +------------------------+ |@@session.tx_isolation | +------------------------+ |READ-COMMITTED | +------------------------+ 1 row in set (0.00 sec)
不可重复读
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>select * from t5 where id=7; +----+------+------+---------+------+ | id | name | age | content | sex | +----+------+------+---------+------+ | 7 | wwb | 29 | dba | M | +----+------+------+---------+------+ 1 row in set (0.00 sec)
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select * from t5 where id=7; +----+------+------+---------+------+ | id | name | age | content | sex | +----+------+------+---------+------+ | 7 | wwb | 29 | dba | M | +----+------+------+---------+------+ 1 row in set (0.00 sec)
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update t5 set sex=\'W\' where id=7; select * from t5; +----+------+------+---------+------+ | id | name | age | content | sex | +----+------+------+---------+------+ | 7 | wwb | 29 | dba | W | +----+------+------+---------+------+ commit; Query OK, 0 rows affected (0.00 sec)
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select * from t5; +----+------+------+---------+------+ | id | name | age | content | sex | +----+------+------+---------+------+ | 7 | wwb | 29 | dba | W | +----+------+------+---------+------+ 1 row in set (0.00 sec)
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幻读
begin; Query OK, 0 rows affected (0.00 sec) >select * from t5; +------+--------+------+---------+------+ | id | name | age | content | sex | +------+--------+------+---------+------+ | 7 | wwb | 29 | dba | M | | 8 | laoyan | 29 | dba | M | +------+--------+------+---------+------+
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begin; Query OK, 0 rows affected (0.00 sec) >select * from t5; +------+--------+------+---------+------+ | id | name | age | content | sex | +------+--------+------+---------+------+ | 7 | wwb | 29 | dba | M | | 8 | laoyan | 29 | dba | M | +------+--------+------+---------+------+ 2 rows in set (0.00 sec)
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>insert into t5 select 9,\'leilei\',32,\'dba\',\'M\'; Query OK, 1 row affected (0.00 sec) Records: 1 Duplicates: 0 Warnings: 0 root@localhost:mysql3308.sock 03:24:05 [wwb]>select * from t5; +------+--------+------+---------+------+ | id | name | age | content | sex | +------+--------+------+---------+------+ | 7 | wwb | 29 | dba | M | | 8 | laoyan | 29 | dba | M | | 9 | leilei | 32 | dba | M | +------+--------+------+---------+------+ 3 rows in set (0.01 sec)
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select * from t5; +------+--------+------+---------+------+ | id | name | age | content | sex | +------+--------+------+---------+------+ | 7 | wwb | 29 | dba | M | | 8 | laoyan | 29 | dba | M | | 9 | leilei | 32 | dba | M | +------+--------+------+---------+------+ 3 rows in set (0.00 sec)
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>select@@session.tx_isolation; +------------------------+ |@@session.tx_isolation | +------------------------+ |REPEATABLE-READ | +------------------------+ 1 row in set (0.00 sec)
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>begin; Query OK, 0 rows affected (0.00 sec) >select * from t5 where id=7; +----+------+------+---------+------+ | id | name | age | content | sex | +----+------+------+---------+------+ | 7 | wwb | 29 | dba | W | +----+------+------+---------+------+ 1 row in set (0.00 sec)
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>begin; Query OK, 0 rows affected (0.00 sec) >select * from t5 where id=7; +----+------+------+---------+------+ | id | name | age | content | sex | +----+------+------+---------+------+ | 7 | wwb | 29 | dba | W | +----+------+------+---------+------+ 1 row in set (0.00 sec)
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>update t5 set sex=\'M\' where id=7; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 >commit; Query OK, 0 rows affected (0.00 sec)
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>select * from t5 where id=7; +----+------+------+---------+------+ | id | name | age | content | sex | +----+------+------+---------+------+ | 7 | wwb | 29 | dba | W | +----+------+------+---------+------+ 1 row in set (0.00 sec) |
修改查看隔离级别
在my.cnf配置文件中【mysqld】分段中，加入一行
Transaction-isolation=‘READ-COMMITTED’ #默认值是REPEATABLE-READ
在线动态修改
Set【GLOBAL|SESSIION】 TRANSACTION ISOLATION LEVEL READ COMMITTED
查看当前隔离级别
Select @@global.tx_isolation，@@session.tx_isolation，@@tx_isolation；
MySQL默认事务隔离级别是:RR
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