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Snailclimb merged 1 commit into from
Aug 24, 2022
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Update mysql-questions-01.md #1805

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6 changes: 3 additions & 3 deletions docs/database/mysql/mysql-questions-01.md
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Expand Up @@ -430,10 +430,10 @@ SELECT ... LOCK IN SHARE MODE

快照读的情况下，如果读取的记录正在执行 UPDATE/DELETE 操作，读取操作不会因此去等待记录上 X 锁的释放，而是会去读取行的一个快照。

只有在事务隔离级别 RC(读取已提交) 和 RR（可重读）下，InnoDB 才会使用非一致性锁定读：
只有在事务隔离级别 RC(读取已提交) 和 RR（可重读）下，InnoDB 才会使用一致性非锁定读：

- 在 RC 级别下，对于快照数据，非一致性读总是读取被锁定行的最新一份快照数据。
- 在 RR 级别下，对于快照数据，非一致性读总是读取本事务开始时的行数据版本。
- 在 RC 级别下，对于快照数据，一致性非锁定读总是读取被锁定行的最新一份快照数据。
- 在 RR 级别下，对于快照数据，一致性非锁定读总是读取本事务开始时的行数据版本。

快照读比较适合对于数据一致性要求不是特别高且追求极致性能的业务场景。

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