ClickHouse® dictionaries

Dictionaries in ClickHouse® are mappings of key-value pairs that are fully or partially stored in the ClickHouse® server memory.
To access dictionary data, you need to use dedicated functions, which run faster than standard SQL queries. You can only read data in dictionaries. ClickHouse® does not support data writes to a dictionary.

The main advantage of dictionaries is that you can get results faster compared to JOIN operations. Opt for dictionaries if you frequently query references to get values by a key.

As their data sources, dictionaries can use ClickHouse® internal dictionaries or external sources: an HTTP(s) resource or any other DBMS, such as MySQL®, ClickHouse®, Yandex StoreDoc, and PostgreSQL.

Internal dictionariesInternal dictionaries

Managed Service for ClickHouse® contains an embedded geobase dictionary and functions to work with it. With this dictionary, you can:

• Get the name of a region in the required language by the region ID.
• Get the ID of a city, district, state/province, country, and continent by the region ID.
• Check whether one region is part of another.
• Get a chain of parent regions.

For more about the functions for working with internal dictionaries, see this ClickHouse® guide.

If the built-in geobase does not fit your purpose, you can add a custom geobase to ClickHouse®.

External dictionariesExternal dictionaries

External dictionaries can use HTTP(s) resources and other DBMS's, such as MySQL®, ClickHouse®, Yandex StoreDoc, and PostgreSQL, as data sources.

Depending on the dictionary settings, ClickHouse® fully or partially stores dictionaries in RAM, updates them on a regular basis, and dynamically loads the missing values.

To learn how to connect and configure a dictionary using Yandex Managed Service for ClickHouse®, see Connecting external dictionaries.

Use casesUse cases

• Connecting external dictionaries in Managed Service for ClickHouse®

ClickHouse® is a registered trademark of ClickHouse, Inc.