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system-variables: add configuration tip based on units (#7390) #7840

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Dec 6, 2021
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system-variables: add configuration tip based on units (#7390) #7840

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system-variables: add configuration tip based on units
shichun-0415 Nov 2, 2021
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Update system-variables.md
TomShawn Dec 6, 2021
68a698c
Update system-variables.md
TomShawn Dec 6, 2021
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12 changes: 12 additions & 0 deletions system-variables.md
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Expand Up @@ -27,6 +27,18 @@ SET GLOBAL tidb_distsql_scan_concurrency = 10;
> - 在 TiDB 服务器上执行 `SET GLOBAL` 语句后，该更改会立即生效。之后会通知所有 TiDB 服务器刷新其系统变量缓存，该操作会在后台立即开始。由于某些 TiDB 服务器可能会错过通知，系统变量缓存每 30 秒会自动刷新一次。这有助于确保所有服务器都以相同的配置运行。
> - 在 TiDB 中，`GLOBAL` 变量的设置即使重启后也仍然有效。此外，由于应用和连接器通常需要读 MySQL 变量，为了兼容这一需求，在 TiDB 中，部分 MySQL 的变量既可读取也可设置。例如，尽管 JDBC 连接器不依赖于查询缓存 (query cache) 的行为，但仍然可以读取和设置查询缓存。

> **注意：**
>
> 变量取较大值并不总会带来更好的性能。由于大部分变量对单个连接生效，设置变量时，还应考虑正在执行语句的并发连接数量。
>
> 确定安全值时，应考虑变量的单位：
>
> * 如果单位为线程，安全值通常取决于 CPU 核的数量。
> * 如果单位为字节，安全值通常小于系统内存的总量。
> * 如果单位为时间，单位可能为秒或毫秒。
>
> 单位相同的多个变量可能会争夺同一组资源。

## 变量参考

### `allow_auto_random_explicit_insert` <span class="version-mark">从 v4.0.3 版本开始引入</span>
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