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<title>8.13.Pseudo-Types</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="datatype-pseudo"></a>8.13.Pseudo-Types</h2></div></div></div>
<a name="id593542"></a><a name="id593553"></a><a name="id593563"></a><a name="id593573"></a><a name="id593583"></a><a name="id593594"></a><a name="id593604"></a><a name="id593614"></a><a name="id593624"></a><a name="id593634"></a><p> The <span class="productname">PostgreSQL</span> type system contains a
number of special-purpose entries that are collectively called
<em class="firstterm">pseudo-types</em>. A pseudo-type cannot be used as a
column data type, but it can be used to declare a function's
argument or result type. Each of the available pseudo-types is
useful in situations where a function's behavior does not
correspond to simply taking or returning a value of a specific
<acronym class="acronym">SQL</acronym> data type. <a href="datatype-pseudo.html#datatype-pseudotypes-table" title="Table8.20.Pseudo-Types">Table8.20, “Pseudo-Types”</a> lists the existing
pseudo-types.
</p>
<div class="table">
<a name="datatype-pseudotypes-table"></a><p class="title"><b>Table8.20.Pseudo-Types</b></p>
<div class="table-contents"><table summary="Pseudo-Types" border="1">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>Name</th>
<th>Description</th>
</tr></thead>
<tbody>
<tr>
<td><code class="type">any</code></td>
<td>Indicates that a function accepts any input data type whatever.</td>
</tr>
<tr>
<td><code class="type">anyarray</code></td>
<td>Indicates that a function accepts any array data type
(see <a href="extend-type-system.html#extend-types-polymorphic" title="32.2.5.Polymorphic Types">Section32.2.5, “Polymorphic Types”</a>).</td>
</tr>
<tr>
<td><code class="type">anyelement</code></td>
<td>Indicates that a function accepts any data type
(see <a href="extend-type-system.html#extend-types-polymorphic" title="32.2.5.Polymorphic Types">Section32.2.5, “Polymorphic Types”</a>).</td>
</tr>
<tr>
<td><code class="type">cstring</code></td>
<td>Indicates that a function accepts or returns a null-terminated C string.</td>
</tr>
<tr>
<td><code class="type">internal</code></td>
<td>Indicates that a function accepts or returns a server-internal
data type.</td>
</tr>
<tr>
<td><code class="type">language_handler</code></td>
<td>A procedural language call handler is declared to return <code class="type">language_handler</code>.</td>
</tr>
<tr>
<td><code class="type">record</code></td>
<td>Identifies a function returning an unspecified row type.</td>
</tr>
<tr>
<td><code class="type">trigger</code></td>
<td>A trigger function is declared to return <code class="type">trigger.</code>
</td>
</tr>
<tr>
<td><code class="type">void</code></td>
<td>Indicates that a function returns no value.</td>
</tr>
<tr>
<td><code class="type">opaque</code></td>
<td>An obsolete type name that formerly served all the above purposes.</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p> Functions coded in C (whether built-in or dynamically loaded) may be
declared to accept or return any of these pseudo data types. It is up to
the function author to ensure that the function will behave safely
when a pseudo-type is used as an argument type.
</p>
<p> Functions coded in procedural languages may use pseudo-types only as
allowed by their implementation languages. At present the procedural
languages all forbid use of a pseudo-type as argument type, and allow
only <code class="type">void</code> and <code class="type">record</code> as a result type (plus
<code class="type">trigger</code> when the function is used as a trigger). Some also
support polymorphic functions using the types <code class="type">anyarray</code> and
<code class="type">anyelement</code>.
</p>
<p> The <code class="type">internal</code> pseudo-type is used to declare functions
that are meant only to be called internally by the database
system, and not by direct invocation in a <acronym class="acronym">SQL</acronym>
query. If a function has at least one <code class="type">internal</code>-type
argument then it cannot be called from <acronym class="acronym">SQL</acronym>. To
preserve the type safety of this restriction it is important to
follow this coding rule: do not create any function that is
declared to return <code class="type">internal</code> unless it has at least one
<code class="type">internal</code> argument.
</p>
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