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update TTL doc to allow split tasks for utf8 column #18457

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Oct 15, 2024
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update TTL doc to allow split tasks for utf8 column #18457

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update TTL doc to allow split tasks for utf8 colum
lcwangchao Aug 27, 2024
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hfxsd Sep 6, 2024
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lcwangchao Sep 14, 2024
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lcwangchao Sep 14, 2024
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11 changes: 10 additions & 1 deletion time-to-live.md
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Expand Up @@ -243,7 +243,16 @@ TTL 功能能够与 TiDB 的迁移、备份、恢复工具一同使用。
* 不允许在临时表上设置 TTL 属性，包括本地临时表和全局临时表。
* 具有 TTL 属性的表不支持作为外键约束的主表被其他表引用。
* 不保证所有过期数据立即被删除，过期数据被删除的时间取决于后台清理任务的调度周期和调度窗口。
* 对于使用[聚簇索引](/clustered-indexes.md)的表，如果主键的类型不是整数类型或二进制字符串类型，TTL 任务将无法被拆分成多个子任务。这将导致 TTL 任务只能在一个 TiDB 节点上按顺序执行。如果表中的数据量较大，TTL 任务的执行可能会变得缓慢。
* 对于使用[聚簇索引](/clustered-indexes.md)的表，仅以下场景支持将 TTL 任务拆分成多个子任务：
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- 主键或者复合主键的第一列为整数或二进制字符串类型。其中，二进制字符串类型主要指下面几种：
- `CHAR(N) CHARACTER SET BINARY`
- `VARCHAR(N) CHARACTER SET BINARY`
- `BINARY(N)`
- `VARBINARY(N)`
- `BIT(N)`
- 主键或者复合主键的第一列的字符集为 `utf8` 或者 `utf8mb4` 且 COLLATE 设置为 `utf8_bin`、 `utf8mb4_bin` 或者 `utf8mb4_0900_bin`。
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* 对于不支持拆分 TTL 子任务的表，TTL 任务只能在一个 TiDB 节点上按顺序执行。此时如果表中的数据量较大，TTL 任务的执行可能会变得缓慢。
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* 对于主键第一列类型是 `utf8` 或者 `utf8mb4` 的表，仅会根据 ASCII 可见字符的范围进行子任务拆分。如果大量的主键值具有相同的 ASCII 前缀，可能会造成任务拆分的不均匀。
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## 常见问题

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