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对设置的限制

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。
约束定义如下:

  1. <profiles>
  2.   <user_name>
  3.     <constraints>
  4.       <setting_name_1>
  5.         <min>lower_boundary</min>
  6.       </setting_name_1>
  7.       <setting_name_2>
  8.         <max>upper_boundary</max>
  9.       </setting_name_2>
  10.       <setting_name_3>
  11.         <min>lower_boundary</min>
  12.         <max>upper_boundary</max>
  13.       </setting_name_3>
  14.       <setting_name_4>
  15.         <readonly/>
  16.       </setting_name_4>
  17.     </constraints>
  18.   </user_name>
  19. </profiles>

如果用户试图违反约束，将引发异常，并且设置不会更改。
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。

示例: 让 users.xml 包括行:

  1. <profiles>
  2.   <default>
  3.     <max_memory_usage>10000000000</max_memory_usage>
  4.     <force_index_by_date>0</force_index_by_date>
  5.     ...
  6.     <constraints>
  7.       <max_memory_usage>
  8.         <min>5000000000</min>
  9.         <max>20000000000</max>
  10.       </max_memory_usage>
  11.       <force_index_by_date>
  12.         <readonly/>
  13.       </force_index_by_date>
  14.     </constraints>
  15.   </default>
  16. </profiles>

以下查询都会引发异常:

  1. SET max_memory_usage=20000000001;
  2. SET max_memory_usage=4999999999;
  3. SET force_index_by_date=1;
  1. Code: 452, e.displayText() = DB::Exception: Setting max_memory_usage should not be greater than 20000000000.
  2. Code: 452, e.displayText() = DB::Exception: Setting max_memory_usage should not be less than 5000000000.
  3. Code: 452, e.displayText() = DB::Exception: Setting force_index_by_date should not be changed.

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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