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97 lines
2.9 KiB
Markdown
97 lines
2.9 KiB
Markdown
---
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sidebar_label: Settings Overview
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sidebar_position: 1
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slug: /en/operations/settings/
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pagination_next: en/operations/settings/settings
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---
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# Settings Overview
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There are multiple ways to define ClickHouse settings. Settings are configured in layers, and each subsequent layer redefines the previous values of a setting.
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The order of priority for defining a setting is:
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1. Settings in the `users.xml` server configuration file
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- Set in the element `<profiles>`.
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2. Session settings
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- Send `SET setting=value` from the ClickHouse console client in interactive mode.
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Similarly, you can use ClickHouse sessions in the HTTP protocol. To do this, you need to specify the `session_id` HTTP parameter.
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3. Query settings
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- When starting the ClickHouse console client in non-interactive mode, set the startup parameter `--setting=value`.
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- When using the HTTP API, pass CGI parameters (`URL?setting_1=value&setting_2=value...`).
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- Define settings in the [SETTINGS](../../sql-reference/statements/select/index.md#settings-in-select-query) clause of the SELECT query. The setting value is applied only to that query and is reset to the default or previous value after the query is executed.
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View the [Settings](./settings.md) page for a description of the ClickHouse settings.
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## Converting a Setting to its Default Value
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If you change a setting and would like to revert it back to its default value, set the value to `DEFAULT`. The syntax looks like:
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```sql
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SET setting_name = DEFAULT
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```
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For example, the default value of `max_insert_block_size` is 1048449. Suppose you change its value to 100000:
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```sql
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SET max_insert_block_size=100000;
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SELECT value FROM system.settings where name='max_insert_block_size';
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```
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The response is:
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```response
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┌─value──┐
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│ 100000 │
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└────────┘
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```
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The following command sets its value back to 1048449:
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```sql
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SET max_insert_block_size=DEFAULT;
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SELECT value FROM system.settings where name='max_insert_block_size';
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```
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The setting is now back to its default:
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```response
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┌─value───┐
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│ 1048449 │
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└─────────┘
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```
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## Custom Settings {#custom_settings}
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In addition to the common [settings](../../operations/settings/settings.md), users can define custom settings.
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A custom setting name must begin with one of predefined prefixes. The list of these prefixes must be declared in the [custom_settings_prefixes](../../operations/server-configuration-parameters/settings.md#custom_settings_prefixes) parameter in the server configuration file.
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```xml
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<custom_settings_prefixes>custom_</custom_settings_prefixes>
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```
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To define a custom setting use `SET` command:
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```sql
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SET custom_a = 123;
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```
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To get the current value of a custom setting use `getSetting()` function:
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```sql
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SELECT getSetting('custom_a');
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```
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**See Also**
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- [Server Configuration Settings](../../operations/server-configuration-parameters/settings.md)
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