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<!--
doc/src/sgml/ref/create_transform.sgml
PostgreSQL documentation
-->
<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><literal><replaceable>from_sql_function_name</replaceable>[(<replaceable>argument_type</replaceable> [, ...])]</literal></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 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>
|
