<type>internal</type>.  The
       actual argument will be of the type for the transform, and the function
       should be coded as if it were.  (But it is not allowed to declare an
       SQL-level function returning <type>internal</type> without at
       least one argument of type <type>internal</type>.)  The actual return
       value will be something specific to the language implementation.
       If no argument list is specified, the function name must be unique in
       its schema.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal><replaceable>to_sql_function_name</replaceable>[(<replaceable>argument_type</replaceable> [, ...])]</literal></term>

     <listitem>
      <para>
       The name of the function for converting the type from the language to
       the SQL environment.  It must take one argument of type
       <type>internal</type> and return the type that is the type for the
       transform.  The actual argument value will be something specific to the
       language implementation.
       If no argument list is specified, the function name must be unique in
       its schema.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
 </refsect1>

 <refsect1 id="sql-createtransform-notes">
  <title>Notes</title>

  <para>
   Use <link linkend="sql-droptransform"><command>DROP TRANSFORM</command></link> to remove transforms.
  </para>
 </refsect1>

 <refsect1 id="sql-createtransform-examples">
  <title>Examples</title>

  <para>
   To create a transform for type <type>hstore</type> and language
   <literal>plpython3u</literal>, first set up the type and the language:
<programlisting>
CREATE TYPE hstore ...;

CREATE EXTENSION plpython3u;
</programlisting>
   Then create the necessary functions:
<programlisting>
CREATE FUNCTION hstore_to_plpython(val internal) RETURNS internal
LANGUAGE C STRICT IMMUTABLE
AS ...;

CREATE FUNCTION plpython_to_hstore(val internal) RETURNS hstore
LANGUAGE C STRICT IMMUTABLE
AS ...;
</programlisting>
   And finally create the transform to connect them all together:
<programlisting>
CREATE TRANSFORM FOR hstore LANGUAGE plpython3u (
    FROM SQL WITH FUNCTION hstore_to_plpython(internal),
    TO SQL WITH FUNCTION plpython_to_hstore(internal)
);
</programlisting>
   In practice, these commands would be wrapped up in an extension.
  </para>

  <para>
   The <filename>contrib</filename> section contains a number of extensions
   that provide transforms, which can serve as real-world examples.
  </para>
 </refsect1>

 <refsect1 id="sql-createtransform-compat">
  <title>Compatibility</title>

  <para>
   This form of <command>CREATE TRANSFORM</command> is a
   <productname>PostgreSQL</productname> extension.  There is a <command>CREATE
   TRANSFORM</command> command in the <acronym>SQL</acronym> standard, but it
   is for adapting data types to client languages.  That usage is not supported
   by <productname>PostgreSQL</productname>.
  </para>
 </refsect1>

 <refsect1 id="sql-createtransform-seealso">
  <title>See Also</title>

  <para>
   <xref linkend="sql-createfunction"/>,
   <xref linkend="sql-createlanguage"/>,
   <xref linkend="sql-createtype"/>,
   <xref linkend="sql-droptransform"/>
  </para>
 </refsect1>

</refentry>