<!-- doc/src/sgml/ref/create_transform.sgml -->

<refentry id="SQL-CREATETRANSFORM">
 <indexterm zone="sql-createtransform">
  <primary>CREATE TRANSFORM</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>CREATE TRANSFORM</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE TRANSFORM</refname>
  <refpurpose>define a new transform</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
CREATE [ OR REPLACE ] TRANSFORM FOR <replaceable>type_name</replaceable> LANGUAGE <replaceable>lang_name</replaceable> (
    FROM SQL WITH FUNCTION <replaceable>from_sql_function_name</replaceable> (<replaceable>argument_type</replaceable> [, ...]),
    TO SQL WITH FUNCTION <replaceable>to_sql_function_name</replaceable> (<replaceable>argument_type</replaceable> [, ...])
);
</synopsis>
 </refsynopsisdiv>

 <refsect1 id="sql-createtransform-description">
  <title>Description</title>

  <para>
   <command>CREATE TRANSFORM</command> defines a new transform.
   <command>CREATE OR REPLACE TRANSFORM</command> will either create a new
   transform, or replace an existing definition.
  </para>

  <para>
   A transform specifies how to adapt a data type to a procedural language.
   For example, when writing a function in PL/Python using
   the <type>hstore</type> type, PL/Python has no prior knowledge how to
   present <type>hstore</type> values in the Python environment.  Language
   implementations usually default to using the text representation, but that
   is inconvenient when, for example, an associative array or a list would be
   more appropriate.
  </para>

  <para>
   A transform specifies two functions:
   <itemizedlist>
    <listitem>
     <para>
      A <quote>from SQL</quote> function that converts the type from the SQL
      environment to the language.  This function will be invoked on the
      arguments of a function written in the language.
     </para>
    </listitem>

    <listitem>
     <para>
      A <quote>to SQL</quote> function that converts the type from the
      language to the SQL environment.  This function will be invoked on the
      return value of a function written in the language.
     </para>
    </listitem>
   </itemizedlist>
   It is not necessary to provide both of these functions.  If one is not
   specified, the language-specific default behavior will be used if
   necessary.  (To prevent a transformation in a certain direction from
   happening at all, you could also write a transform function that always
   errors out.)
  </para>

  <para>
   To be able to create a transform, you must own and
   have <literal>USAGE</literal> privilege on the type, have
   <literal>USAGE</literal> privilege on the language, and own and
   have <literal>EXECUTE</literal> privilege on the from-SQL and to-SQL
   functions, if specified.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

   <variablelist>
    <varlistentry>
     <term><replaceable>type_name</replaceable></term>

     <listitem>
      <para>
       The name of the data type of the transform.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>lang_name</replaceable></term>

     <listitem>
      <para>
       The name of the language of the transform.
      </para>
     </listitem>
    </varlistentry>

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

     <listitem>
      <para>
       The name of the function for converting the type from the SQL
       environment to the language.  It must take one argument of
       type <type>internal</type> and return type <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 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.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>to_sql_function_name</replaceable>(<replaceable>argument_type</replaceable> [, ...])</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.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
 </refsect1>

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

  <para>
   Use <xref linkend="sql-droptransform"> 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>plpythonu</literal>, first set up the type and the language:
<programlisting>
CREATE TYPE hstore ...;

CREATE LANGUAGE plpythonu ...;
</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 plpythonu (
    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 extensions.
  </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>
