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A <em class="firstterm">domain</em> is a user-defined data type that is
based on another <em class="firstterm">underlying type</em>. 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.
</p><p>
For example, we could create a domain over integers that accepts only
positive integers:
</p><pre class="programlisting">
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
</pre><p>
</p><p>
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 <code class="literal">mytable.id - 1</code>
is considered to be of type <code class="type">integer</code> not <code class="type">posint</code>.
We could write <code class="literal">(mytable.id - 1)::posint</code> to cast the
result back to <code class="type">posint</code>, 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 <code class="structfield">id</code> 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.
</p><p>
For additional information see <a class="xref" href="sql-createdomain.html" title="CREATE DOMAIN"><span class="refentrytitle">CREATE DOMAIN</span></a>.
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