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<title>Chapter35.Procedural Languages</title>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="chapter" lang="en" id="xplang">
<div class="titlepage"><div><div><h2 class="title">
<a name="xplang"></a>Chapter35.Procedural Languages</h2></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl><dt><span class="sect1"><a href="xplang.html#xplang-install">35.1. Installing Procedural Languages</a></span></dt></dl>
</div>
<a name="id719856"></a><p> <span class="productname">PostgreSQL</span> allows user-defined functions
to be written in other languages besides SQL and C. These other
languages are generically called <em class="firstterm">procedural
languages</em> (<acronym class="acronym">PL</acronym>s). For a function
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 “<span class="quote">glue</span>” between
<span class="productname">PostgreSQL</span> and an existing implementation
of a programming language. The handler itself is a
C language function compiled into a shared object and
loaded on demand, just like any other C function.
</p>
<p> There are currently four procedural languages available in the
standard <span class="productname">PostgreSQL</span> distribution:
<span class="application">PL/pgSQL</span> (<a href="plpgsql.html" title="Chapter36.PL/pgSQL - SQL Procedural Language">Chapter36, <i><span class="application">PL/pgSQL</span> - <acronym class="acronym">SQL</acronym> Procedural Language</i></a>),
<span class="application">PL/Tcl</span> (<a href="pltcl.html" title="Chapter37.PL/Tcl - Tcl Procedural Language">Chapter37, <i>PL/Tcl - Tcl Procedural Language</i></a>),
<span class="application">PL/Perl</span> (<a href="plperl.html" title="Chapter38.PL/Perl - Perl Procedural Language">Chapter38, <i>PL/Perl - Perl Procedural Language</i></a>), and
<span class="application">PL/Python</span> (<a href="plpython.html" title="Chapter39.PL/Python - Python Procedural Language">Chapter39, <i>PL/Python - Python Procedural Language</i></a>).
Other languages can be defined by users.
The basics of developing a new procedural language are covered in <a href="plhandler.html" title="Chapter46.Writing A Procedural Language Handler">Chapter46, <i>Writing A Procedural Language Handler</i></a>.
</p>
<p> There are additional procedural languages available that are not
included in the core distribution. <a href="external-projects.html" title="AppendixH.External Projects">AppendixH, <i>External Projects</i></a>
has information about finding them.
</p>
<div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="xplang-install"></a>35.1.Installing Procedural Languages</h2></div></div></div>
<p> A procedural language must be “<span class="quote">installed</span>” into each
database where it is to be used. But procedural languages installed in
the database <code class="literal">template1</code> are automatically available in all
subsequently created databases, since their entries in
<code class="literal">template1</code> will be copied by <code class="command">CREATE DATABASE</code>.
So the database administrator can
decide which languages are available in which databases and can make
some languages available by default if he chooses.
</p>
<p> For the languages supplied with the standard distribution, it is
only necessary to execute <code class="command">CREATE LANGUAGE</code>
<em class="replaceable"><code>language_name</code></em> to install the language into the
current database. Alternatively, the program <a href="app-createlang.html" title="createlang"><span class="refentrytitle"><a name="app-createlang-title"></a><span class="application">createlang</span></span></a> may be used to do this from the shell
command line. For example, to install the language
<span class="application">PL/pgSQL</span> into the database
<code class="literal">template1</code>, use
</p>
<pre class="programlisting">createlang plpgsql template1</pre>
<p>
The manual procedure described below is only recommended for
installing custom languages that <code class="command">CREATE LANGUAGE</code>
does not know about.
</p>
<div class="procedure">
<a name="id720085"></a><p class="title"><b> Manual Procedural Language Installation
</b></p>
<p> A procedural language is installed in a database in four steps,
which must be carried out by a database superuser. (For languages
known to <code class="command">CREATE LANGUAGE</code>, the second and third steps
can be omitted, because they will be carried out automatically
if needed.)
</p>
<ol type="1">
<li>
<a name="xplang-install-cr1"></a><p> 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 <a href="xfunc-c.html#dfunc" title="32.9.6.Compiling and Linking Dynamically-Loaded Functions">Section32.9.6, “Compiling and Linking Dynamically-Loaded Functions”</a>. Often, the language
handler will depend on an external library that provides the actual
programming language engine; if so, that must be installed as well.
</p>
</li>
<li>
<a name="xplang-install-cr2"></a><p> The handler must be declared with the command
</p>
<pre class="synopsis">CREATE FUNCTION <em class="replaceable"><code>handler_function_name</code></em>()
RETURNS language_handler
AS '<em class="replaceable"><code>path-to-shared-object</code></em>'
LANGUAGE C;</pre>
<p>
The special return type of <code class="type">language_handler</code> tells
the database system that this function does not return one of
the defined <acronym class="acronym">SQL</acronym> data types and is not directly usable
in <acronym class="acronym">SQL</acronym> statements.
</p>
</li>
<li>
<a name="xplang-install-cr3"></a><p> Optionally, the language handler may provide a “<span class="quote">validator</span>”
function that checks a function definition for correctness without
actually executing it. The validator function is called by
<code class="command">CREATE FUNCTION</code> if it exists. If a validator function
is provided by the handler, declare it with a command like
</p>
<pre class="synopsis">CREATE FUNCTION <em class="replaceable"><code>validator_function_name</code></em>(oid)
RETURNS void
AS '<em class="replaceable"><code>path-to-shared-object</code></em>'
LANGUAGE C;</pre>
<p>
</p>
</li>
<li>
<a name="xplang-install-cr4"></a><p> The PL must be declared with the command
</p>
<pre class="synopsis">CREATE [<span class="optional">TRUSTED</span>] [<span class="optional">PROCEDURAL</span>] LANGUAGE <em class="replaceable"><code>language-name</code></em>
HANDLER <em class="replaceable"><code>handler_function_name</code></em>
[<span class="optional">VALIDATOR <em class="replaceable"><code>validator_function_name</code></em></span>] ;</pre>
<p>
The optional key word <code class="literal">TRUSTED</code> 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 <code class="literal">TRUSTED</code> flag should only be given
for languages that do not allow access to database server
internals or the file system. The languages
<span class="application">PL/pgSQL</span>,
<span class="application">PL/Tcl</span>, and
<span class="application">PL/Perl</span>
are considered trusted; the languages
<span class="application">PL/TclU</span>,
<span class="application">PL/PerlU</span>, and
<span class="application">PL/PythonU</span>
are designed to provide unlimited functionality and should
<span class="emphasis"><em>not</em></span> be marked trusted.
</p>
</li>
</ol>
</div>
<p> <a href="xplang.html#xplang-install-example" title="Example35.1.Manual Installation of PL/pgSQL">Example35.1, “Manual Installation of <span class="application">PL/pgSQL</span>”</a> shows how the manual
installation procedure would work with the language
<span class="application">PL/pgSQL</span>.
</p>
<div class="example">
<a name="xplang-install-example"></a><p class="title"><b>Example35.1.Manual Installation of <span class="application">PL/pgSQL</span></b></p>
<div class="example-contents">
<p> The following command tells the database server where to find the
shared object for the <span class="application">PL/pgSQL</span> language's call handler function.
</p>
<pre class="programlisting">CREATE FUNCTION plpgsql_call_handler() RETURNS language_handler AS
'$libdir/plpgsql' LANGUAGE C;</pre>
<p>
</p>
<p> <span class="application">PL/pgSQL</span> has a validator function,
so we declare that too:
</p>
<pre class="programlisting">CREATE FUNCTION plpgsql_validator(oid) RETURNS void AS
'$libdir/plpgsql' LANGUAGE C;</pre>
<p>
</p>
<p> The command
</p>
<pre class="programlisting">CREATE TRUSTED PROCEDURAL LANGUAGE plpgsql
HANDLER plpgsql_call_handler
VALIDATOR plpgsql_validator;</pre>
<p>
then defines that the previously declared functions
should be invoked for functions and trigger procedures where the
language attribute is <code class="literal">plpgsql</code>.
</p>
</div>
</div>
<br class="example-break"><p> In a default <span class="productname">PostgreSQL</span> installation,
the handler for the <span class="application">PL/pgSQL</span> language
is built and installed into the “<span class="quote">library</span>”
directory. If <span class="application">Tcl</span> support is configured in, the handlers
for <span class="application">PL/Tcl</span> and <span class="application">PL/TclU</span> are also built and
installed in the same location. Likewise, the <span class="application">PL/Perl</span> and
<span class="application">PL/PerlU</span> handlers are built and installed if Perl support
is configured, and the <span class="application">PL/PythonU</span> handler is
installed if Python support is configured.
</p>
</div>
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