Merge pull request #15368 from olgarev/revolg-DOCSUP-2403-Document_the_mechanics_of_custom_settings_specifying

DOCSUP-2403: Docs for the mechanics of custom settings specifying

docs/en/operations/server-configuration-parameters/settings.md

@@ -66,6 +66,20 @@ If no conditions met for a data part, ClickHouse uses the `lz4` compression.
 </compression>
 ```
 
+## custom_settings_prefixes {#custom_settings_prefixes}
+
+List of prefixes for [custom settings](../../operations/settings/index.md#custom_settings). The prefixes must be separated with commas.
+
+**Example**
+
+```xml
+<custom_settings_prefixes>custom_</custom_settings_prefixes>
+```
+
+**See Also**
+
+-   [Custom settings](../../operations/settings/index.md#custom_settings)
+
 ## core_dump
 
 Configures soft limit for core dump file size, one gigabyte by default.

docs/en/operations/settings/index.md

@@ -28,4 +28,30 @@ Ways to configure settings, in order of priority:
 
 Settings that can only be made in the server config file are not covered in this section.
 
+## Custom Settings {#custom_settings}
+
+In addition to the common [settings](../../operations/settings/settings.md), 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](../../operations/server-configuration-parameters/settings.md#custom_settings_prefixes) parameter in the server configuration file.
+
+```xml
+<custom_settings_prefixes>custom_</custom_settings_prefixes>
+```
+
+To define a custom setting use `SET` command:
+
+```sql
+SET custom_a = 123;
+```
+
+To get the current value of a custom setting use `getSetting()` function:
+
+```sql
+SELECT getSetting('custom_a');    
+```
+
+**See Also**
+
+-   [Server Configuration Settings](../../operations/server-configuration-parameters/settings.md)
+
 [Original article](https://clickhouse.tech/docs/en/operations/settings/) <!--hide-->

docs/en/sql-reference/functions/other-functions.md

@@ -1491,4 +1491,40 @@ Result:
 
 ```
 
+## getSetting {#getSetting}
+
+Returns the current value of a [custom setting](../../operations/settings/index.md#custom_settings).
+
+**Syntax** 
+
+```sql
+getSetting('custom_setting');    
+```
+
+**Parameter** 
+
+-   `custom_setting` — The setting name. [String](../../sql-reference/data-types/string.md).
+
+**Returned value**
+
+-   The setting current value.
+
+**Example**
+
+```sql
+SET custom_a = 123;
+SELECT getSetting('custom_a');    
+```
+
+**Result**
+
+```
+123
+```
+
+**See Also** 
+
+-   [Custom Settings](../../operations/settings/index.md#custom_settings)
+
+
 [Original article](https://clickhouse.tech/docs/en/query_language/functions/other_functions/) <!--hide-->