Split off language-specific docs to their own chapters.

Add PL/perl docs from Mark Hollomon.
Put language-specific docs into the User's Guide, and move the rest
 to the Programmer's Guide.

doc/src/sgml/plperl.sgml

+ <chapter id="pl-perl">
+  <title>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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