ClickHouse/docs/en/query_language/dicts/external_dicts.md

# 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 "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 {#ext-dicts-see-also}

- [Configuring an External Dictionary](external_dicts_dict.md)
- [Storing Dictionaries in Memory](external_dicts_dict_layout.md)
- [Dictionary Updates](external_dicts_dict_lifetime.md)
- [Sources of External Dictionaries](external_dicts_dict_sources.md)
- [Dictionary Key and Fields](external_dicts_dict_structure.md)
- [Functions for Working with External Dictionaries](../functions/ext_dict_functions.md)

[Original article](https://clickhouse.tech/docs/en/query_language/dicts/external_dicts/) <!--hide-->