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 `integer`

not `posint`

.
We could write `(mytable.id - 1)::posint`

to cast the
result back to `posint`

, 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 `id`

value of
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.