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110 lines
4.5 KiB
Markdown
110 lines
4.5 KiB
Markdown
---
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slug: /en/operations/settings/constraints-on-settings
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sidebar_position: 62
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sidebar_label: Constraints on Settings
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---
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# Constraints on Settings
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The constraints on settings can be defined in the `profiles` section of the `user.xml` configuration file and prohibit users from changing some of the settings with the `SET` query.
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The constraints are defined as the following:
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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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<setting_name_5>
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<min>lower_boundary</min>
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<max>upper_boundary</max>
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<changeable_in_readonly/>
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</setting_name_5>
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</constraints>
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</user_name>
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</profiles>
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```
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If the user tries to violate the constraints an exception is thrown and the setting isn’t changed.
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There are supported few types of constraints: `min`, `max`, `readonly` (with alias `const`) and `changeable_in_readonly`. The `min` and `max` constraints specify upper and lower boundaries for a numeric setting and can be used in combination. The `readonly` or `const` constraint specifies that the user cannot change the corresponding setting at all. The `changeable_in_readonly` constraint type allows user to change the setting within `min`/`max` range even if `readonly` setting is set to 1, otherwise settings are not allow to be changed in `readonly=1` mode. Note that `changeable_in_readonly` is supported only if `settings_constraints_replace_previous` is enabled:
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``` xml
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<access_control_improvements>
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<settings_constraints_replace_previous>true</settings_constraints_replace_previous>
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</access_control_improvements>
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```
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If there are multiple profiles active for a user, then constraints are merged. Merge process depends on `settings_constraints_replace_previous`:
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- **true** (recommended): constraints for the same setting are replaced during merge, such that the last constraint is used and all previous are ignored including fields that are not set in new constraint.
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- **false** (default): constraints for the same setting are merged in a way that every not set type of constraint is taken from previous profile and every set type of constraint is replaced by value from new profile.
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Read-only mode is enabled by `readonly` setting (not to confuse with `readonly` constraint type):
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- `readonly=0`: No read-only restrictions.
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- `readonly=1`: Only read queries are allowed and settings cannot be changed unless `changeable_in_readonly` is set.
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- `readonly=2`: Only read queries are allowed, but settings can be changed, except for `readonly` setting itself.
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**Example:** Let `users.xml` includes lines:
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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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The following queries all throw exceptions:
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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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**Note:** the `default` profile has special handling: all the constraints defined for the `default` profile become the default constraints, so they restrict all the users until they’re overridden explicitly for these users.
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## Constraints on Merge Tree Settings
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It is possible to set constraints for [merge tree settings](merge-tree-settings.md). These constraints are applied when table with merge tree engine is created or its storage settings are altered. Name of merge tree setting must be prepended by `merge_tree_` prefix when referenced in `<constraints>` section.
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**Example:** Forbid to create new tables with explicitly specified `storage_policy`
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``` xml
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<profiles>
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<default>
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<constraints>
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<merge_tree_storage_policy>
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<const/>
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</merge_tree_storage_policy>
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</constraints>
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</default>
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</profiles>
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```
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