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Domains

Domains are special-purpose types that add some extra features atop of existing base type, but leaving on-wire and on-disc format of the underlying data type intact. At the moment, ClickHouse does not support user-defined domains.

You can use domains anywhere corresponding base type can be used, for example:

  • Create a column of a domain type
  • Read/write values from/to domain column
  • Use it as an index if a base type can be used as an index
  • Call functions with values of domain column

Extra Features of Domains​

  • Explicit column type name in SHOW CREATE TABLE or DESCRIBE TABLE
  • Input from human-friendly format with INSERT INTO domain_table(domain_column) VALUES(...)
  • Output to human-friendly format for SELECT domain_column FROM domain_table
  • Loading data from an external source in the human-friendly format: INSERT INTO domain_table FORMAT CSV ...

Limitations​

  • Can't convert index column of base type to domain type via ALTER TABLE.
  • Can't implicitly convert string values into domain values when inserting data from another column or table.
  • Domain adds no constrains on stored values.