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 <chapter id="plperl">
  <title>PL/Perl - Perl Procedural Language</title>

  <indexterm zone="plperl">
   <primary>PL/Perl</primary>
  </indexterm>

  <indexterm zone="plperl">
   <primary>Perl</primary>
  </indexterm>

  <para>
   PL/Perl is a loadable procedural language
   that enables the <ulink url="http://www.perl.com">Perl</ulink> programming
   language to be used to write 
   <productname>PostgreSQL</productname> functions.
  </para>

  <!-- **** PL/Perl overview **** -->

  <sect1 id="plperl-overview">
   <title>Overview</title>

   <para>
    Normally, PL/Perl is installed as a <quote>trusted</> programming
    language named <literal>plperl</>.  In this setup, certain Perl
    operations are disabled to preserve security.  In general, the operations
    that are restricted are those that interact with the environment. This
    includes file handle operations, <literal>require</literal>, and
    <literal>use</literal> (for external modules).
    There is no way to access internals of the
    database backend or to gain OS-level access under the permissions of the
    <productname>PostgreSQL</productname> user ID, as a C function can do.
    Thus, any unprivileged database user may be
    permitted to use this language.
   </para>
   <para>
    Sometimes it is desirable to write Perl functions that are not restricted
    --- for example, one might want a Perl function that sends
    mail.  To handle these cases, PL/Perl can also be installed as an
    <quote>untrusted</> language (usually named <literal>plperlu</>).
    In this case the full Perl language is available.  The writer of a PL/PerlU
    function must take care that the function cannot be used to do anything
    unwanted, since it will be able to do anything that could be done by
    a user logged in as the database administrator.  Note that the database
    system allows only database superusers to create functions in untrusted
    languages.
   </para>
 </sect1>

 <sect1 id="plperl-install">
  <title>Building and Installing PL/Perl</title>

  <para>
   If the <option>--with-perl</option> option was supplied to the
   <indexterm><primary><filename>configure</filename></primary></indexterm>
   <filename>configure</filename> script,
   the <productname>PostgreSQL</productname> build process will attempt to
   build the PL/Perl shared library and install it in the 
   <productname>PostgreSQL</productname> library directory.
  </para>

  <para>
   On most platforms, since PL/Perl is a shared library, the
   <indexterm><primary>libperl</primary></indexterm>
   <filename>libperl</filename> library must be a shared library also.
   At the time of this writing, this is almost never the case in prebuilt
   Perl packages.  If this difficulty arises in your situation, a
   message like this will appear during the build to point out this
   fact:
<screen>
<computeroutput>
*** Cannot build PL/Perl because libperl is not a shared library.
*** You might have to rebuild your Perl installation.  Refer to
*** the documentation for details.
</computeroutput>
</screen>
   If you see this, you will have to re-build and install
   <productname>Perl</productname> manually to be able to build
   PL/Perl.  During the configuration process for
   <productname>Perl</productname>, request a shared library.
  </para>

  <para>
   After having reinstalled Perl, change to the directory
   <filename>src/pl/plperl</filename> in the
   <productname>PostgreSQL</productname> source tree and issue the commands
<programlisting>
gmake clean
gmake all
gmake install
</programlisting>
   to complete the build and installation of the PL/Perl shared library.
  </para>

   <para>
    To install
    PL/Perl and/or PL/PerlU in a particular database, use the
    <filename>createlang</filename> script, for example
    <literal>createlang plperl <replaceable>dbname</></literal> or
    <literal>createlang plperlu <replaceable>dbname</></literal>.
   </para>

  <tip>
   <para>
    If a language is installed into <literal>template1</>, all subsequently
    created databases will have the language installed automatically.
   </para>
  </tip>
  </sect1>

  <!-- **** PL/Perl description **** -->

  <sect1 id="plperl-description">
   <title>Description</title>

   <sect2>
    <title>PL/Perl Functions and Arguments</title>

    <para>
     To create a function in the PL/Perl language, use the standard syntax

     <programlisting>
CREATE FUNCTION <replaceable>funcname</replaceable> (<replaceable>argument-types</replaceable>) RETURNS <replaceable>return-type</replaceable> AS '
    # PL/Perl function body
' LANGUAGE plperl;
     </programlisting>

     PL/PerlU is the same, except that the language should be specified as
     <literal>plperlu</>.
    </para>

    <para>
     The body of the function is ordinary Perl code.  Arguments and
     results are handled as in any other Perl subroutine: arguments
     are passed in <varname>@_</varname>, and a result value is returned
     with <literal>return</> or as the last expression evaluated in the
     function.  For example, a function
     returning the greater of two integer values could be defined as:

     <programlisting>
CREATE FUNCTION perl_max (integer, integer) RETURNS integer AS '
    if ($_[0] > $_[1]) { return $_[0]; }
    return $_[1];
' LANGUAGE plperl;
     </programlisting>

     If a NULL is passed to a function, the argument value will appear
     as <quote>undefined</> in Perl.  The above function definition will
     not behave very nicely with NULL inputs (in fact, it will act as
     though they are zeroes).  We could add <literal>WITH (isStrict)</>
     to the function definition to make <productname>PostgreSQL</productname>
     do something more reasonable: if a NULL is passed, the
     function will not be called at all, but will just return a NULL
     result automatically.  Alternatively, we could check for undefined
     inputs in the function body.  For example, suppose that we wanted perl_max
     with one null and one non-null argument to return the non-null
     argument, rather than NULL:

     <programlisting>
CREATE FUNCTION perl_max (integer, integer) RETURNS integer AS '
    my ($a,$b) = @_;
    if (! defined $a) {
        if (! defined $b) { return undef; }
        return $b;
    }
    if (! defined $b) { return $a; }
    if ($a > $b) { return $a; }
    return $b;
' LANGUAGE plperl;
     </programlisting>
    </para>

    <para>
     As shown above,
     to return a NULL from a PL/Perl function, return an undefined
     value.  This can be done whether the function is strict or not.
    </para>

    <para>
     Composite-type arguments are passed to the function as references to
     hashes.  The keys of the hash are the attribute names of the composite
     type.  Here is an example:

     <programlisting>
CREATE TABLE employee (
    name text,
    basesalary integer,
    bonus integer
);

CREATE FUNCTION empcomp(employee) RETURNS integer AS '
    my ($emp) = @_;
    return $emp->{''basesalary''} + $emp->{''bonus''};
' LANGUAGE plperl;

SELECT name, empcomp(employee) FROM employee;
     </programlisting>
    </para>

    <para>
     There is not currently any support for returning a composite-type
     result value.
    </para>

  <tip>
   <para>
    Because the function body is passed as an SQL string literal to
    <command>CREATE FUNCTION</command>, you have to escape single
    quotes and backslashes within your Perl source, typically by doubling them
    as shown in the above example.  Another possible approach is to
    avoid writing single quotes by using Perl's extended quoting functions
    (<literal>q[]</literal>, <literal>qq[]</literal>,
    <literal>qw[]</literal>).
   </para>
  </tip>

  <para>
   Here is an example of a function that will not work because file
   system operations are not allowed for security reasons:
<programlisting>
CREATE FUNCTION badfunc() RETURNS integer AS '
    open(TEMP, ">/tmp/badfile");
    print TEMP "Gotcha!\n";
    return 1;
' LANGUAGE plperl;
</programlisting>
   The creation of the function will succeed, but executing it will not.
  </para>
  <para>
   Note that if the same function was created by a superuser using language 
   <literal>plperlu</>, execution would succeed.
  </para>

   </sect2>

   <sect2>
    <title>Data Values in PL/Perl</title>

    <para>
     The argument values supplied to a PL/Perl function's script are simply
     the input arguments converted to text form (just as if they had been
     displayed by a SELECT statement).  Conversely, the <literal>return</>
     command will accept any string that is acceptable input format for
     the function's declared return type.  So, the PL/Perl programmer can
     manipulate data values as if they were just text.
    </para>

   </sect2>

   <sect2>
    <title>Database Access from PL/Perl</title>

  <para>
   Access to the database itself from your Perl function can be done via
   an experimental module <ulink
   url="http://www.cpan.org/modules/by-module/DBD/APILOS/"><literal>DBD::PgSPI</literal></ulink>
   (also available at <ulink url="http://www.cpan.org/SITES.html">CPAN
   mirror sites</ulink>). This module makes available a
   <acronym>DBI</>-compliant database-handle named
   <varname>$pg_dbh</varname> that can be used to perform queries
   with normal <acronym>DBI</> syntax.
  </para>

    <para>
     PL/Perl itself presently provides only one additional Perl command:
    </para>

    <variablelist>
     <varlistentry>
      <indexterm>
       <primary>elog</primary>
      </indexterm>
      <term><function>elog</> <replaceable>level</replaceable>, <replaceable>msg</replaceable></term>
      <listitem>
       <para>
	Emit a log or error message. Possible levels are <literal>DEBUG</>,
	<literal>NOTICE</>, and <literal>ERROR</>.
	<literal>DEBUG</> and <literal>NOTICE</> simply emit the given message
	into the postmaster log (and send it to the client too, in the case of
	<literal>NOTICE</>).  <literal>ERROR</> raises an error condition:
	further execution of the function is abandoned, and the current
	transaction is aborted.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>

   </sect2>

   <sect2>
    <title>Missing Features</title>

    <para>
     PL/Perl functions cannot call each other directly (because they
     are anonymous subroutines inside Perl).  There's presently
     no way for them to share global variables, either.
    </para>

    <para>
     PL/Perl cannot currently be used to write trigger functions.
    </para>

    <para>
     DBD::PgSPI or similar capability should be integrated
     into the standard <productname>PostgreSQL</productname> distribution.
    </para>

   </sect2>

  </sect1>
 </chapter>

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