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| slug | sidebar_position | sidebar_label |
|---|---|---|
| /en/sql-reference/dictionaries/external-dictionaries/external-dicts | 39 | General Description |
import CloudDetails from '@site/docs/en/sql-reference/dictionaries/external-dictionaries/_snippet_dictionary_in_cloud.md';
Dictionaries
:::tip Tutorial If you are getting started with Dictionaries in ClickHouse we have a tutorial that covers that topic. Take a look here. :::
You can add your own dictionaries from various data sources. The source for a dictionary can be a ClickHouse table, a local text or executable file, an HTTP(s) resource, or another DBMS. For more information, see “Dictionary Sources”.
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 creating dictionaries with xml files or DDL queries.
The configuration of dictionaries can be located in one or more xml-files. The path to the configuration is specified in the dictionaries_config parameter.
Dictionaries can be loaded at server startup or at first use, depending on the dictionaries_lazy_load setting.
The dictionaries system table contains information about dictionaries configured at server. For each dictionary you can find there:
- Status of the dictionary.
- Configuration parameters.
- Metrics like amount of RAM allocated for the dictionary or a number of queries since the dictionary was successfully loaded.
Creating a dictionary with a DDL query
Dictionaries can be created with DDL queries, and this is the recommended method because with DDL created dictionaries:
- No additional records are added to server configuration files
- The dictionaries can be worked with as first-class entities, like tables or views
- Data can be read directly, using familiar SELECT rather than dictionary table functions
- The dictionaries can be easily renamed
Creating a dictionary with a configuration file
:::note
Creating a dictionary with a configuration file is not applicable to ClickHouse Cloud. Please use DDL (see above), and create your dictionary as user default.
:::
The dictionary configuration file has the following format:
<clickhouse>
<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>
</clickhouse>
You can configure any number of dictionaries in the same file.
:::note
You can convert values for a small dictionary by describing it in a SELECT query (see the transform function). This functionality is not related to dictionaries.
:::