ClickHouse/docs/en/query_language/dicts/external_dicts.md
2020-01-30 13:34:55 +03:00

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# External Dictionaries {#dicts-external_dicts}
You can add your own dictionaries from various data sources. The data source for a dictionary can be a local text or executable file, an HTTP(s) resource, or another DBMS. For more information, see "[Sources for external dictionaries](external_dicts_dict_sources.md)".
ClickHouse:
- Fully or partially stores dictionaries in RAM.
- Periodically updates dictionaries and dynamically loads missing values. In other words, dictionaries can be loaded dynamically.
- Allows to create external dictionaries with xml-files or [DDL queries](../create.md#create-dictionary-query).
The configuration of external dictionaries can be located in one or more xml-files. The path to the configuration is specified in the [dictionaries_config](../../operations/server_settings/settings.md#server_settings-dictionaries_config) parameter.
Dictionaries can be loaded at server startup or at first use, depending on the [dictionaries_lazy_load](../../operations/server_settings/settings.md#server_settings-dictionaries_lazy_load) setting.
The dictionary configuration file has the following format:
```xml
<yandex>
<comment>An optional element with any content. Ignored by the ClickHouse server.</comment>
<!--Optional element. File name with substitutions-->
<include_from>/etc/metrika.xml</include_from>
<dictionary>
<!-- Dictionary configuration. -->
<!-- There can be any number of <dictionary> sections in the configuration file. -->
</dictionary>
</yandex>
```
You can [configure](external_dicts_dict.md) any number of dictionaries in the same file.
[DDL queries for dictionaries](../create.md#create-dictionary-query) doesn't require any additional records in server configuration. They allow to work with dictionaries as first-class entities, like tables or views.
!!! attention
You can convert values for a small dictionary by describing it in a `SELECT` query (see the [transform](../functions/other_functions.md) function). This functionality is not related to external dictionaries.
**See also**
- [Functions for Working with External Dictionaries](../functions/ext_dict_functions.md)
[Original article](https://clickhouse.tech/docs/en/query_language/dicts/external_dicts/) <!--hide-->