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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 Vsnapshot" /><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 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 15.6 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 /></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="link" href="sql-droptransform.html" title="DROP TRANSFORM"><code class="command">DROP TRANSFORM</code></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">plpython3u</code>, first set up the type and the language:
</p><pre class="programlisting">
CREATE TYPE hstore ...;

CREATE EXTENSION plpython3u;
</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 plpython3u (
    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>
  </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-createtstemplate.html" title="CREATE TEXT SEARCH TEMPLATE">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-createtrigger.html" title="CREATE TRIGGER">Next</a></td></tr><tr><td width="40%" align="left" valign="top">CREATE TEXT SEARCH TEMPLATE </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.6 Documentation">Home</a></td><td width="40%" align="right" valign="top"> CREATE TRIGGER</td></tr></table></div></body></html>