Domains

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

You can use domains anywhere corresponding base type can be used:

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

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 external source in 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.