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update transaction doc (#6858) #7036

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Aug 30, 2021
Merged

update transaction doc (#6858) #7036

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12a4e32
update transaction doc
you06 Aug 13, 2021
189b23c
update
you06 Aug 13, 2021
7e821df
a tiny fix
TomShawn Aug 30, 2021
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2 changes: 1 addition & 1 deletion optimistic-transaction.md
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Expand Up @@ -64,7 +64,7 @@ summary: 了解 TiDB 的乐观事务模型。

## 事务的重试

使用乐观事务模型时，在高冲突率的场景中，事务容易发生写写冲突而导致提交失败。MySQL 使用悲观事务模型，在执行 SQL 语句的过程中进行加锁并且其 Repeatable Read 隔离级别允许出现不可重复读，所以提交时一般不会出现异常。为了降低应用改造难度，TiDB 提供了数据库内部自动重试机制。
使用乐观事务模型时，在高冲突率的场景中，事务容易发生写写冲突而导致提交失败。MySQL 使用悲观事务模型，在执行写入类型的 SQL 语句的过程中进行加锁并且在 Repeatable Read 隔离级别下使用了当前读的机制，能够读取到最新的数据，所以提交时一般不会出现异常。为了降低应用改造难度，TiDB 提供了数据库内部自动重试机制。

### 重试机制

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2 changes: 1 addition & 1 deletion transaction-isolation-levels.md
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Expand Up @@ -49,7 +49,7 @@ commit; |

### 与 MySQL 可重复读隔离级别的区别

MySQL 可重复读隔离级别在更新时并不检验当前版本是否可见，也就是说，即使该行在事务启动后被更新过，同样可以继续更新。这种情况在 TiDB 会导致事务回滚，导致事务最终失败，而 MySQL 是可以更新成功的。MySQL 的可重复读隔离级别并非 Snapshot 隔离级别，MySQL 可重复读隔离级别的一致性要弱于 Snapshot 隔离级别，也弱于 TiDB 的可重复读隔离级别。
MySQL 可重复读隔离级别在更新时并不检验当前版本是否可见，也就是说，即使该行在事务启动后被更新过，同样可以继续更新。这种情况在 TiDB 使用乐观事务时会导致事务回滚，导致事务最终失败，而 TiDB 默认的悲观事务和 MySQL 是可以更新成功的。

## 读已提交隔离级别 (Read Committed)

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