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 <chapter id="xplang">
  <title id="xplang-title">Procedural Languages</title>

  <indexterm zone="xplang">
   <primary>procedural language</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> allows user-defined functions
   to be written in other languages besides SQL and C.  These other
   languages are generically called <firstterm>procedural
   languages</firstterm> (<acronym>PL</>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 <quote>glue</quote> between
   <productname>PostgreSQL</productname> 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.
  </para>

  <para>
   There are currently four procedural languages available in the
   standard <productname>PostgreSQL</productname> distribution:
   <application>PL/pgSQL</application> (<xref linkend="plpgsql">),
   <application>PL/Tcl</application> (<xref linkend="pltcl">),
   <application>PL/Perl</application> (<xref linkend="plperl">), and
   <application>PL/Python</application> (<xref linkend="plpython">).
   Other languages can be defined by users.
   The basics of developing a new procedural language are covered in <xref
   linkend="plhandler">.
  </para>

  <para>
   There are additional procedural languages available that are not
   included in the core distribution. <xref linkend="external-projects">
   has information about finding them.
  </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, since their entries in
    <literal>template1</> will be copied by <command>CREATE DATABASE</>.
    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, it is
    only necessary to execute <command>CREATE LANGUAGE</>
    <replaceable>language_name</> to install the language into the
    current database.  Alternatively, the program <xref
    linkend="app-createlang"> may be used to do this from the shell
    command line.  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>CREATE LANGUAGE</command>
    does not know about.
   </para>

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

    <para>
     A procedural language is installed in a database in four steps,
     which must be carried out by a database superuser.  (For languages
     known to <command>CREATE LANGUAGE</>, the second and third steps
     can be omitted, because they will be carried out automatically
     if needed.)
    </para>

    <step performance="required" id="xplang-install-cr1">
     <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">.  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.
     </para>
    </step>

    <step performance="required" id="xplang-install-cr2">
     <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="optional" id="xplang-install-cr3">
     <para>
      Optionally, the language handler may provide a <quote>validator</>
      function that checks a function definition for correctness without
      actually executing it.  The validator function is called by
      <command>CREATE FUNCTION</> if it exists.  If a validator function
      is provided by the handler, declare it with a command like
<synopsis>
CREATE FUNCTION <replaceable>validator_function_name</replaceable>(oid)
    RETURNS void
    AS '<replaceable>path-to-shared-object</replaceable>'
    LANGUAGE C;
</synopsis>
     </para>
    </step>

    <step performance="required" id="xplang-install-cr4">
     <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>
    <optional>VALIDATOR <replaceable>validator_function_name</replaceable></optional> ;
</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>
      <application>PL/pgSQL</application> has a validator function,
      so we declare that too:

<programlisting>
CREATE FUNCTION plpgsql_validator(oid) RETURNS void AS
    '$libdir/plpgsql' LANGUAGE C;
</programlisting>
     </para>

     <para>
      The command
<programlisting>
CREATE TRUSTED PROCEDURAL LANGUAGE plpgsql
    HANDLER plpgsql_call_handler
    VALIDATOR plpgsql_validator;
</programlisting>
      then defines that the previously declared functions
      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</> 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 the <application>PL/PythonU</> handler is
    installed if Python support is configured.
   </para>

  </sect1>

</chapter>

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