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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>CREATE TRANSFORM</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="sql-createtstemplate.html" title="CREATE TEXT SEARCH TEMPLATE" /><link rel="next" href="sql-createtrigger.html" title="CREATE TRIGGER" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">CREATE TRANSFORM</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-createtstemplate.html" title="CREATE TEXT SEARCH TEMPLATE">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-createtrigger.html" title="CREATE TRIGGER">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SQL-CREATETRANSFORM"><div class="titlepage"></div><a id="id-1.9.3.92.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">CREATE TRANSFORM</span></h2><p>CREATE TRANSFORM — define a new transform</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">
CREATE [ OR REPLACE ] TRANSFORM FOR <em class="replaceable"><code>type_name</code></em> LANGUAGE <em class="replaceable"><code>lang_name</code></em> (
FROM SQL WITH FUNCTION <em class="replaceable"><code>from_sql_function_name</code></em> [ (<em class="replaceable"><code>argument_type</code></em> [, ...]) ],
TO SQL WITH FUNCTION <em class="replaceable"><code>to_sql_function_name</code></em> [ (<em class="replaceable"><code>argument_type</code></em> [, ...]) ]
);
</pre></div><div class="refsect1" id="SQL-CREATETRANSFORM-DESCRIPTION"><h2>Description</h2><p>
<code class="command">CREATE TRANSFORM</code> defines a new transform.
<code class="command">CREATE OR REPLACE TRANSFORM</code> will either create a new
transform, or replace an existing definition.
</p><p>
A transform specifies how to adapt a data type to a procedural language.
For example, when writing a function in PL/Python using
the <code class="type">hstore</code> type, PL/Python has no prior knowledge how to
present <code class="type">hstore</code> 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.
</p><p>
A transform specifies two functions:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
A <span class="quote">“<span class="quote">from SQL</span>”</span> 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.
</p></li><li class="listitem"><p>
A <span class="quote">“<span class="quote">to SQL</span>”</span> 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.
</p></li></ul></div><p>
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.)
</p><p>
To be able to create a transform, you must own and
have <code class="literal">USAGE</code> privilege on the type, have
<code class="literal">USAGE</code> privilege on the language, and own and
have <code class="literal">EXECUTE</code> privilege on the from-SQL and to-SQL
functions, if specified.
</p></div><div class="refsect1" id="id-1.9.3.92.6"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>type_name</code></em></span></dt><dd><p>
The name of the data type of the transform.
</p></dd><dt><span class="term"><em class="replaceable"><code>lang_name</code></em></span></dt><dd><p>
The name of the language of the transform.
</p></dd><dt><span class="term"><code class="literal"><em class="replaceable"><code>from_sql_function_name</code></em>[(<em class="replaceable"><code>argument_type</code></em> [, ...])]</code></span></dt><dd><p>
The name of the function for converting the type from the SQL
environment to the language. It must take one argument of
type <code class="type">internal</code> and return type <code class="type">internal</code>. 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 <code class="type">internal</code> without at
least one argument of type <code class="type">internal</code>.) 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.
</p></dd><dt><span class="term"><code class="literal"><em class="replaceable"><code>to_sql_function_name</code></em>[(<em class="replaceable"><code>argument_type</code></em> [, ...])]</code></span></dt><dd><p>
The name of the function for converting the type from the language to
the SQL environment. It must take one argument of type
<code class="type">internal</code> 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.
</p></dd></dl></div></div><div class="refsect1" id="SQL-CREATETRANSFORM-NOTES"><h2>Notes</h2><p>
Use <a class="xref" href="sql-droptransform.html" title="DROP TRANSFORM"><span class="refentrytitle">DROP TRANSFORM</span></a> to remove transforms.
</p></div><div class="refsect1" id="SQL-CREATETRANSFORM-EXAMPLES"><h2>Examples</h2><p>
To create a transform for type <code class="type">hstore</code> and language
<code class="literal">plpythonu</code>, first set up the type and the language:
</p><pre class="programlisting">
CREATE TYPE hstore ...;
CREATE EXTENSION plpythonu;
</pre><p>
Then create the necessary functions:
</p><pre class="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 ...;
</pre><p>
And finally create the transform to connect them all together:
</p><pre class="programlisting">
CREATE TRANSFORM FOR hstore LANGUAGE plpythonu (
FROM SQL WITH FUNCTION hstore_to_plpython(internal),
TO SQL WITH FUNCTION plpython_to_hstore(internal)
);
</pre><p>
In practice, these commands would be wrapped up in an extension.
</p><p>
The <code class="filename">contrib</code> section contains a number of extensions
that provide transforms, which can serve as real-world examples.
</p></div><div class="refsect1" id="SQL-CREATETRANSFORM-COMPAT"><h2>Compatibility</h2><p>
This form of <code class="command">CREATE TRANSFORM</code> is a
<span class="productname">PostgreSQL</span> extension. There is a <code class="command">CREATE
TRANSFORM</code> command in the <acronym class="acronym">SQL</acronym> standard, but it
is for adapting data types to client languages. That usage is not supported
by <span class="productname">PostgreSQL</span>.
</p></div><div class="refsect1" id="SQL-CREATETRANSFORM-SEEALSO"><h2>See Also</h2><p>
<a class="xref" href="sql-createfunction.html" title="CREATE FUNCTION"><span class="refentrytitle">CREATE FUNCTION</span></a>,
<a class="xref" href="sql-createlanguage.html" title="CREATE LANGUAGE"><span class="refentrytitle">CREATE LANGUAGE</span></a>,
<a class="xref" href="sql-createtype.html" title="CREATE TYPE"><span class="refentrytitle">CREATE TYPE</span></a>,
<a class="xref" href="sql-droptransform.html" title="DROP TRANSFORM"><span class="refentrytitle">DROP TRANSFORM</span></a>
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