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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/xplang.sgml,v 1.12 2001/02/09 02:20:52 tgl Exp $
-->
<chapter id="xplang">
<title id="xplang-title">Procedural Languages</title>
<para>
<productname>Postgres</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>Postgres</productname> and an existing implementation
of a programming language. The handler itself is a special
programming language function compiled into a shared object and
loaded on demand.
</para>
<para>
Writing a handler for a new procedural language is outside the
scope of this manual, although some information is provided in
the CREATE LANGUAGE reference page. Several procedural languages are
available in the standard <productname>Postgres</productname> distribution.
</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 template1 database 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
shell script <filename>createlang</filename> may be used instead
of carrying out the details by hand. For example, to install PL/pgSQL
into the template1 database, use
<programlisting>
createlang plpgsql template1
</programlisting>
The manual procedure described below is only recommended for
installing custom languages that <filename>createlang</filename>
does not know about.
</para>
<procedure>
<title>
Manual Procedural Language Installation
</title>
<para>
A procedural language is installed in the database in three
steps, which must be carried out by a database superuser.
</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">
<para>
The handler must be declared with the command
<synopsis>
CREATE FUNCTION <replaceable>handler_function_name</replaceable> ()
RETURNS OPAQUE AS
'<replaceable>path-to-shared-object</replaceable>' LANGUAGE 'C';
</synopsis>
The special return type of <type>OPAQUE</type> tells
the database 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">
<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>
LANCOMPILER '<replaceable>description</replaceable>';
</synopsis>
The optional key word <token>TRUSTED</token> tells
whether 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 backend, the <acronym>TRUSTED</acronym>
flag should only be given for
languages that do not allow access to database backends
internals or the filesystem. The languages PL/pgSQL,
PL/Tcl, and PL/Perl are known to be trusted; the language PL/TclU
should <emphasis>not</emphasis> be marked trusted.
</para>
</step>
</procedure>
<para>
In a default <productname>Postgres</productname> installation, the
handler for the PL/pgSQL language is built and installed into the
<quote>library</quote> directory. If Tcl/Tk support is configured
in, the handlers for PL/Tcl and PL/TclU are also built and installed in
the same location. Likewise, the PL/Perl handler is built and installed
if Perl support is configured. The <filename>createlang</filename>
script automates the two CREATE steps described above.
</para>
<procedure>
<title>Example</title>
<step performance="required">
<para>
The following command tells the database where to find the
shared object for the PL/pgSQL language's call handler function.
<programlisting>
CREATE FUNCTION plpgsql_call_handler () RETURNS OPAQUE AS
'/usr/local/pgsql/lib/plpgsql.so' LANGUAGE 'C';
</programlisting>
</para>
</step>
<step performance="Required">
<para>
The command
<programlisting>
CREATE TRUSTED PROCEDURAL LANGUAGE 'plpgsql'
HANDLER plpgsql_call_handler
LANCOMPILER 'PL/pgSQL';
</programlisting>
then defines that the previously declared call handler function
should be invoked for functions and trigger procedures where the
language attribute is 'plpgsql'.
</para>
</step>
</procedure>
</sect1>
</chapter>
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