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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/xplang.sgml,v 1.25 2003/10/22 22:28:10 tgl Exp $
-->
<chapter id="xplang">
<title id="xplang-title">Procedural Languages</title>
<indexterm zone="xplang">
<primary>procedural language</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname> allows users to add new
programming languages to be available for writing functions and
procedures. These are called <firstterm>procedural
languages</firstterm> (PL). In the case of a function or trigger
procedure written in a procedural language, the database server has
no built-in knowledge about how to interpret the function's source
text. Instead, the task is passed to a special handler that knows
the details of the language. The handler could either do all the
work of parsing, syntax analysis, execution, etc. itself, or it
could serve as <quote>glue</quote> between
<productname>PostgreSQL</productname> and an existing implementation
of a programming language. The handler itself is a special
C language function compiled into a shared object and
loaded on demand.
</para>
<para>
Writing a handler for a new procedural language is described in
<xref linkend="plhandler">. Several procedural languages are
available in the standard <productname>PostgreSQL</productname>
distribution, which can serve as examples.
</para>
<sect1 id="xplang-install">
<title>Installing Procedural Languages</title>
<para>
A procedural language must be <quote>installed</quote> into each
database where it is to be used. But procedural languages installed in
the database <literal>template1</> are automatically available in all
subsequently created databases. So the database administrator can
decide which languages are available in which databases and can make
some languages available by default if he chooses.
</para>
<para>
For the languages supplied with the standard distribution, the
program <command>createlang</command> may be used to install the
language instead of carrying out the details by hand. For
example, to install the language
<application>PL/pgSQL</application> into the database
<literal>template1</>, use
<programlisting>
createlang plpgsql template1
</programlisting>
The manual procedure described below is only recommended for
installing custom languages that <command>createlang</command>
does not know about.
</para>
<procedure>
<title>
Manual Procedural Language Installation
</title>
<para>
A procedural language is installed in a database in three steps,
which must be carried out by a database superuser. The
<command>createlang</command> program automates <xref
linkend="xplang-install-cr1"> and <xref
linkend="xplang-install-cr2">.
</para>
<step performance="required">
<para>
The shared object for the language handler must be compiled and
installed into an appropriate library directory. This works in the same
way as building and installing modules with regular user-defined C
functions does; see <xref linkend="dfunc">.
</para>
</step>
<step performance="required" id="xplang-install-cr1">
<para>
The handler must be declared with the command
<synopsis>
CREATE FUNCTION <replaceable>handler_function_name</replaceable>()
RETURNS language_handler
AS '<replaceable>path-to-shared-object</replaceable>'
LANGUAGE C;
</synopsis>
The special return type of <type>language_handler</type> tells
the database system that this function does not return one of
the defined <acronym>SQL</acronym> data types and is not directly usable
in <acronym>SQL</acronym> statements.
</para>
</step>
<step performance="required" id="xplang-install-cr2">
<para>
The PL must be declared with the command
<synopsis>
CREATE <optional>TRUSTED</optional> <optional>PROCEDURAL</optional> LANGUAGE <replaceable>language-name</replaceable>
HANDLER <replaceable>handler_function_name</replaceable>;
</synopsis>
The optional key word <literal>TRUSTED</literal> specifies that
ordinary database users that have no superuser privileges should
be allowed to use this language to create functions and trigger
procedures. Since PL functions are executed inside the database
server, the <literal>TRUSTED</literal> flag should only be given
for languages that do not allow access to database server
internals or the file system. The languages
<application>PL/pgSQL</application>,
<application>PL/Tcl</application>, and
<application>PL/Perl</application>
are considered trusted; the languages
<application>PL/TclU</application>,
<application>PL/PerlU</application>, and
<application>PL/PythonU</application>
are designed to provide unlimited functionality and should
<emphasis>not</emphasis> be marked trusted.
</para>
</step>
</procedure>
<para>
<xref linkend="xplang-install-example"> shows how the manual
installation procedure would work with the language
<application>PL/pgSQL</application>.
</para>
<example id="xplang-install-example">
<title>Manual Installation of <application>PL/pgSQL</application></title>
<para>
The following command tells the database server where to find the
shared object for the <application>PL/pgSQL</application> language's call handler function.
<programlisting>
CREATE FUNCTION plpgsql_call_handler() RETURNS language_handler AS
'$libdir/plpgsql' LANGUAGE C;
</programlisting>
</para>
<para>
The command
<programlisting>
CREATE TRUSTED PROCEDURAL LANGUAGE plpgsql
HANDLER plpgsql_call_handler;
</programlisting>
then defines that the previously declared call handler function
should be invoked for functions and trigger procedures where the
language attribute is <literal>plpgsql</literal>.
</para>
</example>
<para>
In a default <productname>PostgreSQL</productname> installation,
the handler for the <application>PL/pgSQL</application> language
is built and installed into the <quote>library</quote>
directory. If <application>Tcl/Tk</> support is configured in, the handlers for
<application>PL/Tcl</> and <application>PL/TclU</> are also built and installed in the same
location. Likewise, the <application>PL/Perl</> and <application>PL/PerlU</> handlers are built
and installed if Perl support is configured, and <application>PL/PythonU</> is
installed if Python support is configured.
</para>
</sect1>
</chapter>
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