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60 lines
3.5 KiB
Markdown
60 lines
3.5 KiB
Markdown
---
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toc_priority: 39
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toc_title: General Description
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---
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# External Dictionaries {#dicts-external-dicts}
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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](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-sources.md)”.
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ClickHouse:
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- Fully or partially stores dictionaries in RAM.
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- Periodically updates dictionaries and dynamically loads missing values. In other words, dictionaries can be loaded dynamically.
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- Allows to create external dictionaries with xml files or [DDL queries](../../../sql-reference/statements/create/dictionary.md).
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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-configuration-parameters/settings.md#server_configuration_parameters-dictionaries_config) parameter.
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Dictionaries can be loaded at server startup or at first use, depending on the [dictionaries_lazy_load](../../../operations/server-configuration-parameters/settings.md#server_configuration_parameters-dictionaries_lazy_load) setting.
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The [dictionaries](../../../operations/system-tables/dictionaries.md#system_tables-dictionaries) system table contains information about dictionaries configured at server. For each dictionary you can find there:
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- Status of the dictionary.
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- Configuration parameters.
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- Metrics like amount of RAM allocated for the dictionary or a number of queries since the dictionary was successfully loaded.
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The dictionary configuration file has the following format:
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``` xml
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<yandex>
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<comment>An optional element with any content. Ignored by the ClickHouse server.</comment>
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<!--Optional element. File name with substitutions-->
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<include_from>/etc/metrika.xml</include_from>
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<dictionary>
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<!-- Dictionary configuration. -->
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<!-- There can be any number of <dictionary> sections in the configuration file. -->
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</dictionary>
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</yandex>
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```
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You can [configure](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict.md) any number of dictionaries in the same file.
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[DDL queries for dictionaries](../../../sql-reference/statements/create/dictionary.md) does not require any additional records in server configuration. They allow to work with dictionaries as first-class entities, like tables or views.
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!!! attention "Attention"
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You can convert values for a small dictionary by describing it in a `SELECT` query (see the [transform](../../../sql-reference/functions/other-functions.md) function). This functionality is not related to external dictionaries.
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## See Also {#ext-dicts-see-also}
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- [Configuring an External Dictionary](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict.md)
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- [Storing Dictionaries in Memory](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-layout.md)
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- [Dictionary Updates](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-lifetime.md)
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- [Sources of External Dictionaries](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-sources.md)
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- [Dictionary Key and Fields](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-structure.md)
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- [Functions for Working with External Dictionaries](../../../sql-reference/functions/ext-dict-functions.md)
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