8.18. Domain Types
A domain is a user-defined data type that is based on another underlying type . Optionally, it can have constraints that restrict its valid values to a subset of what the underlying type would allow. Otherwise it behaves like the underlying type - for example, any operator or function that can be applied to the underlying type will work on the domain type. The underlying type can be any built-in or user-defined base type, enum type, array type, composite type, range type, or another domain.
For example, we could create a domain over integers that accepts only positive integers:
CREATE DOMAIN posint AS integer CHECK (VALUE > 0); CREATE TABLE mytable (id posint); INSERT INTO mytable VALUES(1); -- works INSERT INTO mytable VALUES(-1); -- fails
When an operator or function of the underlying type is applied to a
domain value, the domain is automatically down-cast to the underlying
type. Thus, for example, the result of
mytable.id - 1
is considered to be of type
We could write
(mytable.id - 1)::posint
to cast the
result back to
, causing the domain's constraints
to be rechecked. In this case, that would result in an error if the
expression had been applied to an
1. Assigning a value of the underlying type to a field or variable of
the domain type is allowed without writing an explicit cast, but the
domain's constraints will be checked.
For additional information see CREATE DOMAIN .