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

  <para>
   This chapter describes how to compile, install and
   use PL/Perl.
  </para>
  <sect1>
   <title>Overview</title>
   <para>
    PL/Perl allows you to write functions in the Perl scripting
    language which may be used in SQL queries as if they were 
    built into <productname>Postgres</productname>. 
   </para>
   <para>
    The PL/Perl intepreter is a full Perl interpreter. However,
    certain operations have been disabled in order to increase
    the security level of the system.
   </para>
   <para>
    In general, the operations that are restricted are those that
    interact with the environment. This includes filehandle operations,
    <literal>require</literal>, and <literal>use</literal> (for external
    modules).
   </para>
   <para>
    It should be noted that this security is not absolute. Indeed, several
    Denial-of-Service attacks are still possible - memory exhaustion and
    endless loops are two.
   </para>
  </sect1>

  <sect1>
   <title>Building and Installing</title>
   <para>
    Assuming that the <productname>Postgres</productname>
    source tree is rooted at $PGSRC, then the Pl/perl source
    code is located in $PGSRC/src/pl/plperl.
   </para>
   <para>
    To build, simply do the following:
    <programlisting>
cd $PGSRC/src/pl/plperl
perl Makefile.PL
make
    </programlisting>
   </para>

   <para>
    This will create a shared library file. On a Linux system, it will be
    named plperl.so. The extension may differ on other systems.
   </para>
   <para>
    The shared library should then be copied into the lib subdirectory of the
    postgres installation.
   </para>
   <para>
    The createlang command is used to install the language into a database.
    If it is installed into template1, all future databases will have the
    language installed automatically.
   </para>
  </sect1>

  <sect1>
   <title>Using PL/Perl</title>
   <para>
    Assume you have the following table:
    <programlisting>
CREATE TABLE EMPLOYEE (
    name text,
    basesalary int4,
    bonus int4 );
    </programlisting>

    In order to get the total compensation (base + bonus) we could
    define a function as follows:
    <programlisting>
CREATE FUNCTION totalcomp(int4, int4) RETURNS int4
    AS 'return $_[0] + $_[1]'
    LANGUAGE 'plperl';
    </programlisting>

    Note that the arguments are passed to the function in
    <literal>@_</literal> as might be expected. Also, because
    of the quoting rules for the SQL creating the function, you
    may find yourself using the extended quoting functions (qq[],
    q[], qw[]) more often that you are used to.
   </para>
   <para>
    We may now use our function like so:
    <programlisting>
SELECT name, totalcomp(basesalary, bonus) from employee
    </programlisting>
   </para>
   <para>
    But, we can also pass entire tuples to our function:
    <programlisting>
CREATE FUNCTION empcomp(employee) returns int4
    AS 'my $emp = shift;
        return $emp->{'basesalary'} + $emp->{'bonus'};'
    LANGUAGE 'plperl';
    </programlisting>
    A tuple is passed as a reference to hash. The keys are the names of
    fields in the tuples. The values are values of the corresponding
    field in the tuple.
   </para>
   <para>
    The new function <literal>empcomp</literal> can used like:
    <programlisting>
SELECT name, empcomp(employee) from employee;
    </programlisting>
   </para>
  </sect1>
 </chapter>

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