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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
orDESCRIBE 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.