ClickHouse/docs/en/operations/settings/index.md

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Settings Overview	51	/en/operations/settings/	en/operations/settings/settings

Settings Overview

There are multiple ways to define ClickHouse settings. Settings are configured in layers, and each subsequent layer redefines the previous values of a setting.

The order of priority for defining a setting is:

1. Settings in the users.xml server configuration file

   • Set in the element <profiles>.

2. Session settings

   • Send SET setting=value from the ClickHouse console client in interactive mode. Similarly, you can use ClickHouse sessions in the HTTP protocol. To do this, you need to specify the session_id HTTP parameter.

3. Query settings

   • When starting the ClickHouse console client in non-interactive mode, set the startup parameter --setting=value.
   • When using the HTTP API, pass CGI parameters (URL?setting_1=value&setting_2=value...).
   • Define settings in the SETTINGS 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.

View the Settings page for a description of the ClickHouse settings.

Converting a Setting to its Default Value

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:

SET setting_name = DEFAULT

For example, the default value of max_insert_block_size is 1048449. Suppose you change its value to 100000:

SET max_insert_block_size=100000;

SELECT value FROM system.settings where name='max_insert_block_size';

The response is:

┌─value──┐
│ 100000 │
└────────┘

The following command sets its value back to 1048449:

SET max_insert_block_size=DEFAULT;

SELECT value FROM system.settings where name='max_insert_block_size';

The setting is now back to its default:

┌─value───┐
│ 1048449 │
└─────────┘

Custom Settings

In addition to the common settings, users can define custom settings.

A custom setting name must begin with one of predefined prefixes. The list of these prefixes must be declared in the custom_settings_prefixes parameter in the server configuration file.

<custom_settings_prefixes>custom_</custom_settings_prefixes>

To define a custom setting use SET command:

SET custom_a = 123;

To get the current value of a custom setting use getSetting() function:

SELECT getSetting('custom_a');

See Also

• Server Configuration Settings