mysql 提交讀 可重複讀

2021-09-22 23:34:23 字數 4173 閱讀 5759

環境

mysql> select version();

+------------+

| version() |

+------------+

| 5.6.31-log |

+------------+

1 row in set (0.00 sec)

檢視當前資料庫隔離級別

mysql> select @@global.tx_isolation,@@tx_isolation;

+-----------------------+----------------+

| @@global.tx_isolation | @@tx_isolation |

+-----------------------+----------------+

| read-committed | read-committed |

+-----------------------+----------------+

1 row in set (0.00 sec)

檢視測試表結構

mysql> show create table t\g;

*****

*****

*****

*****

*****

** 1. row **

*****

*****

*****

*****

*****

table: t

create table: create table `t` (

`id` int(11) not null auto_increment,

`comment` varchar(30) default null,

`time` datetime default null,

primary key (`id`)

) engine=innodb auto_increment=2 default charset=utf8

1 row in set (0.00 sec)

解決髒讀問題,但是會出現不可重複讀

#session 1                                          session 2

mysql> begin; mysql> begin;

query ok, 0 rows affected (0.00 sec) query ok, 0 rows affected (0.00 sec)

mysql> select * from t;

empty set (0.00 sec)

mysql> insert into t(comment,time) values ('r1',now());

query ok, 1 row affected (0.00 sec)

mysql> commit;

query ok, 0 rows affected (0.00 sec)

mysql> select * from t;

+----+---------+---------------------+

| id | comment | time |

+----+---------+---------------------+

| 1 | r1 | 2016-09-23 15:56:17 |

+----+---------+---------------------+

1 row in set (0.00 sec)

mysql> commit;

query ok, 0 rows affected (0.00 sec)

設定全域性的隔離級別為repeatable-read

mysql>

setglobal tx_isolation=

'repeatable-read';

query ok, 0

rows affected (0.00 sec)

清空測試資料

mysql> truncate table t;

query ok, 0 rows affected (0.03 sec)

重複執行上面的實驗,可見在可重複讀的隔離級別下,不可重複讀得到了解決

#session 1                                          session 2

mysql> begin; mysql> begin;

query ok, 0 rows affected (0.00 sec) query ok, 0 rows affected (0.00 sec)

mysql> select * from t;

empty

set (0.00 sec)

mysql> insert into t(comment,time) values ('r2',now());

query ok, 1 row affected (0.00 sec)

mysql> commit;

query ok, 0 rows affected (0.00 sec)

mysql> select * from t;

empty

set (0.00 sec)

mysql> commit;

query ok, 0 rows affected (0.00 sec)

清空測試資料

mysql> truncate table t;

query ok, 0 rows affected (0.03 sec)

進一步測試,幻讀現象出現

#session 1                                          session 2

mysql> begin; mysql> begin;

query ok, 0 rows affected (0.00 sec) query ok, 0 rows affected (0.00 sec)

mysql> select * from t;

empty set (0.00 sec)

mysql> insert into t(comment,time) values ('r3',now());

query ok, 1 row affected (0.00 sec)

mysql> commit;

query ok, 0 rows affected (0.00 sec)

mysql> select * from t;

empty set (0.00 sec)

mysql> update t set comment='c3';

query ok, 1 row affected (0.00 sec)

rows matched: 1 changed: 1 warnings: 0

mysql> select * from t;

+----+---------+---------------------+

| id | comment | time |

+----+---------+---------------------+

| 1 | c3 | 2016-09-23 16:53:28 |

+----+---------+---------------------+

1 row in set (0.00 sec)

mysql> commit;

query ok, 0 rows affected (0.00 sec)

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