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 <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 a <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.  Several procedural languages are available
   in the <productname>Postgres</productname> distribution.
  </para>

  <sect1 id="xplang-install">
   <title>Installing Procedural Languages</title>

   <procedure>
    <title>
     Procedural Language Installation
    </title>

    <para>
     A procedural language is installed in the database in three
     steps.  A procedural language must be installed into each
     database where it is to be used.  Procedural languages defined in
     the template1 database are automatically available in all
     subsequently created databases. So the administrator can decide
     which languages are available by default.
    </para>

    <step performance="required">
     <para>
      The shared object for the language handler must be compiled and
      installed.  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 and
      PL/Tcl are known to be trusted.
     </para>
    </step>
   </procedure>

   <para>
    In a default <productname>Postgres</productname> installation, the
    handler for the PL/pgSQL is built and installed into the
    <quote>library</quote> directory. If Tcl/Tk support is configured
    in, the handler for PL/Tcl is also built and installed in the same
    location.
   </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>

   <para>
    For the languages supplied with the standard distribution, the
    shell script <filename>createlang</filename> can be used instead
    of carrying out the details manually.  To install PL/pgSQL into
    the template1 database, use
<programlisting>
createlang plpgsql template1
</programlisting>
   </para>

     <para>
      PL handler functions have a special call interface that is
      different from regular C language functions. One of the arguments
      given to the handler is the object ID in the <filename>pg_proc</filename>
      tables entry for the function that should be executed.
      The handler examines various system catalogs to analyze the
      functions call arguments and it's return data type. The source
      text of the functions body is found in the prosrc attribute of
      <literal>pg_proc</literal>.
      Due to this, PL functions
      can be overloaded like SQL language functions. There can be
      multiple different PL functions having the same function name,
      as long as the call arguments differ.
     </para>

  </sect1>

</chapter>

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