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.