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75 lines
2.2 KiB
Markdown
75 lines
2.2 KiB
Markdown
---
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slug: /zh/operations/settings/constraints-on-settings
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machine_translated: true
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machine_translated_rev: 72537a2d527c63c07aa5d2361a8829f3895cf2bd
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sidebar_position: 62
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sidebar_label: "\u5BF9\u8BBE\u7F6E\u7684\u9650\u5236"
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---
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# 对设置的限制 {#constraints-on-settings}
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在设置的约束可以在定义 `profiles` 一节 `user.xml` 配置文件，并禁止用户更改一些设置与 `SET` 查询。
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约束定义如下:
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``` xml
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<profiles>
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<user_name>
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<constraints>
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<setting_name_1>
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<min>lower_boundary</min>
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</setting_name_1>
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<setting_name_2>
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<max>upper_boundary</max>
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</setting_name_2>
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<setting_name_3>
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<min>lower_boundary</min>
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<max>upper_boundary</max>
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</setting_name_3>
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<setting_name_4>
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<readonly/>
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</setting_name_4>
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</constraints>
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</user_name>
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</profiles>
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```
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如果用户试图违反约束，将引发异常，并且设置不会更改。
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支持三种类型的约束: `min`, `max`, `readonly`. 该 `min` 和 `max` 约束指定数值设置的上边界和下边界，并且可以组合使用。 该 `readonly` constraint指定用户根本无法更改相应的设置。
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**示例:** 让 `users.xml` 包括行:
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``` xml
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<profiles>
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<default>
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<max_memory_usage>10000000000</max_memory_usage>
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<force_index_by_date>0</force_index_by_date>
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...
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<constraints>
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<max_memory_usage>
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<min>5000000000</min>
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<max>20000000000</max>
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</max_memory_usage>
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<force_index_by_date>
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<readonly/>
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</force_index_by_date>
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</constraints>
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</default>
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</profiles>
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```
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以下查询都会引发异常:
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``` sql
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SET max_memory_usage=20000000001;
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SET max_memory_usage=4999999999;
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SET force_index_by_date=1;
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```
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``` text
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Code: 452, e.displayText() = DB::Exception: Setting max_memory_usage should not be greater than 20000000000.
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Code: 452, e.displayText() = DB::Exception: Setting max_memory_usage should not be less than 5000000000.
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Code: 452, e.displayText() = DB::Exception: Setting force_index_by_date should not be changed.
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```
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**注:** 该 `default` 配置文件具有特殊的处理：所有定义的约束 `default` 配置文件成为默认约束，因此它们限制所有用户，直到为这些用户显式复盖它们。
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