mysql的读提交与可重复读
前景介绍
| 隔离级别 | 脏读可能性 | 不可重复读可能性 | 幻读可能性 | 加读锁 |
|---|---|---|---|---|
| READ UNCOMMITTED | YES | YES | YES | NO |
| READ COMMITTED | NO | YES | YES | NO |
| REPEATABLE READ | NO | NO | YES | NO |
| SERALIZABLE | NO | NO | NO | NO |
mysql事务
READ COMMITTED
| 时间 | 事务1 | 事务2 | 事务3 |
|---|---|---|---|
| T1 | begin | begin | begin |
| T2 | update wx_va set value = “TT1” where id = 3; | select * from wx_va where id = 3; | |
| T3 | commit | ||
| T4 | select * from wx_va where id = 3; | select * from wx_va where id = 3; | |
| T5 | update wx_va set value = “TT2” where id = 3 | ||
| T6 | commit | select * from wx_va where id = 3; | |
| T7 | commit | ||
| T8 | select* from wx_va where id = 3; |
T4 的时候事务3 读到了提交的数据
T4的时候事务2由于在提交直接读了一遍相同的数据，因此事务2读到的是提交之前的数据。
REPEATABLE READ
| 时间 | 事务1 | 事务2 | 事务3 |
|---|---|---|---|
| T1 | begin | begin | begin |
| T2 | update wx_va set value = “TT1” where id = 3; | select * from wx_va where id = 3; | |
| T3 | commit | ||
| T4 | select * from wx_va where id = 3; | select * from wx_va where id = 3; | |
| T5 | update wx_va set value = “TT2” where id = 3 | ||
| T6 | commit | select * from wx_va where id = 3; | |
| T7 | commit | ||
| T8 | select* from wx_va where id = 3; |
猜想：在每个事务中，无论这个select执行几次，到底是在提交之前执行了还是提交之后执行了，他读到的都是这个begin 开启时的数据。
验证：