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Overhaul plperl documentation.

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Tom Lane 2002-01-25 19:13:15 +00:00
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doc/src/sgml/plperl.sgml
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@ -1,60 +1,75 @@
<!-- <!--
$Header: /cvsroot/pgsql/doc/src/sgml/plperl.sgml,v 2.14 2002/01/08 16:13:41 petere Exp $ $Header: /cvsroot/pgsql/doc/src/sgml/plperl.sgml,v 2.15 2002/01/25 19:13:15 tgl Exp $
--> -->
<chapter id="plperl"> <chapter id="plperl">
<title>PL/Perl - Perl Procedural Language</title> <title>PL/Perl - Perl Procedural Language</title>
<indexterm zone="plperl"> <indexterm zone="plperl">
<primary>PL/Perl</primary> <primary>PL/Perl</primary>
</indexterm> </indexterm>
<indexterm zone="plperl"> <indexterm zone="plperl">
<primary>Perl</primary> <primary>Perl</primary>
</indexterm> </indexterm>
<sect1 id="intro"> <para>
<title>Introduction</title> 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>
<para> <!-- **** PL/Perl overview **** -->
PL/Perl allows you to write functions in the <ulink
url="http://www.perl.com">Perl</ulink> programming language that may
be used in SQL queries as if they were built into
<productname>PostgreSQL</productname>.
</para>
<para> <sect1 id="plperl-overview">
The PL/Perl interpreter (when installed as trusted interpreter with <title>Overview</title>
default name <literal>plperl</>) is a full Perl interpreter. However, certain
operations have been disabled in order to maintain the security of
the system. 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). 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 examples.
</para> <para>
<para> Normally, PL/Perl is installed as a <quote>trusted</> programming
When PL/Perl is installed as <quote>untrusted</> interpreter (with name <literal>plperlu</literal>), language named <literal>plperl</>. In this setup, certain Perl
everything is permitted, and any Perl code can be loaded (by a superuser only). operations are disabled to preserve security. In general, the operations
</para> 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>
<sect1 id="plperl-install"> <sect1 id="plperl-install">
<title>Building and Installing</title> <title>Building and Installing PL/Perl</title>
<para> <para>
In order to build and install PL/Perl if you are installing If the <option>--with-perl</option> option was supplied to the
<productname>PostgreSQL</productname> from source then the
<option>--with-perl</option> must be supplied to the
<indexterm><primary><filename>configure</filename></primary></indexterm> <indexterm><primary><filename>configure</filename></primary></indexterm>
<filename>configure</filename> script. PL/Perl requires that, when <filename>configure</filename> script,
<productname>Perl</productname> was installed, the 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> <indexterm><primary>libperl</primary></indexterm>
<filename>libperl</filename> library was build as a shared object. <filename>libperl</filename> library must be a shared library also.
At the time of this writing, this is almost never the case in the At the time of this writing, this is almost never the case in prebuilt
Perl packages that are distributed with the operating systems. A Perl packages. If this difficulty arises in your situation, a
message like this will appear during the build to point out this message like this will appear during the build to point out this
fact: fact:
<screen> <screen>
@ -64,14 +79,14 @@ $Header: /cvsroot/pgsql/doc/src/sgml/plperl.sgml,v 2.14 2002/01/08 16:13:41 pete
*** the documentation for details. *** the documentation for details.
</computeroutput> </computeroutput>
</screen> </screen>
Therefore it is likely that you will have to re-build and install If you see this, you will have to re-build and install
<productname>Perl</productname> manually to be able to build <productname>Perl</productname> manually to be able to build
PL/Perl. PL/Perl. During the configuration process for
<productname>Perl</productname>, request a shared library.
</para> </para>
<para> <para>
When you want to retry to build PL/Perl after having reinstalled After having reinstalled Perl, change to the directory
Perl, then change to the directory
<filename>src/pl/plperl</filename> in the <filename>src/pl/plperl</filename> in the
<productname>PostgreSQL</productname> source tree and issue the commands <productname>PostgreSQL</productname> source tree and issue the commands
<programlisting> <programlisting>
@ -79,88 +94,131 @@ gmake clean
gmake all gmake all
gmake install gmake install
</programlisting> </programlisting>
to complete the build and installation of the PL/Perl shared library.
</para> </para>
<para> <para>
The <command>createlang</command> command is used to install the To install
language into a database. PL/Perl and/or PL/PerlU in a particular database, use the
<screen> <filename>createlang</filename> script, for example
<prompt>$</prompt> <userinput>createlang plperl template1</userinput> <literal>createlang plperl <replaceable>dbname</></literal> or
</screen> <literal>createlang plperlu <replaceable>dbname</></literal>.
Alternatively, to create untrusted interpreter (where functions can only </para>
be created by a superuser, but the functions are not restricted), use:
<screen>
<prompt>$</prompt> <userinput>createlang plperlu template1</userinput>
</screen>
If it is installed into template1, all future databases will have
the language installed automatically.
</para>
</sect1>
<sect1 id="plperl-use"> <tip>
<title>Using PL/Perl</title> <para>
If a language is installed into <literal>template1</>, all subsequently
created databases will have the language installed automatically.
</para>
</tip>
</sect1>
<para> <!-- **** PL/Perl description **** -->
Assume you have the following table:
<programlisting> <sect1 id="plperl-description">
<title>Description</title>
<sect2>
<title>PL/Perl Functions and Arguments</title>
<para>
To create a function in the PL/Perl language, use the standard syntax
<programlisting>
CREATE FUNCTION <replaceable>funcname</replaceable> (<replaceable>argument-types</replaceable>) RETURNS <replaceable>return-type</replaceable> AS '
# PL/Perl function body
' LANGUAGE plperl;
</programlisting>
PL/PerlU is the same, except that the language should be specified as
<literal>plperlu</>.
</para>
<para>
The body of the function is ordinary Perl code. Arguments and
results are handled as in any other Perl subroutine: arguments
are passed in <varname>@_</varname>, and a result value is returned
with <literal>return</> or as the last expression evaluated in the
function. For example, a function
returning the greater of two integer values could be defined as:
<programlisting>
CREATE FUNCTION perl_max (integer, integer) RETURNS integer AS '
if ($_[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 ( CREATE TABLE employee (
name text, name text,
basesalary integer, basesalary integer,
bonus integer bonus integer
); );
</programlisting>
In order to get the total compensation (base + bonus) we could
define a function as follows:
<programlisting>
CREATE FUNCTION totalcomp(integer, integer) RETURNS integer
AS 'return $_[0] + $_[1]'
LANGUAGE plperl;
</programlisting>
Notice that the arguments to the function are passed in
<varname>@_</varname> as might be expected.
</para>
<para>
We can 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 functions:
<programlisting>
CREATE FUNCTION empcomp(employee) RETURNS integer AS ' CREATE FUNCTION empcomp(employee) RETURNS integer AS '
my $emp = shift; my ($emp) = @_;
return $emp->{''basesalary''} + $emp->{''bonus''}; return $emp->{''basesalary''} + $emp->{''bonus''};
' LANGUAGE plperl; ' LANGUAGE plperl;
</programlisting>
A tuple is passed as a reference to a hash. The keys are the names SELECT name, empcomp(employee) FROM employee;
of the fields in the tuples. The hash values are values of the </programlisting>
corresponding fields in the tuple. </para>
</para>
<para>
There is not currently any support for returning a composite-type
result value.
</para>
<tip> <tip>
<para> <para>
Because the function body is passed as an SQL string literal to Because the function body is passed as an SQL string literal to
<command>CREATE FUNCTION</command> you have to escape single <command>CREATE FUNCTION</command>, you have to escape single
quotes within your Perl source, either by doubling them as shown quotes and backslashes within your Perl source, typically by doubling them
above, or by using the extended quoting functions 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>q[]</literal>, <literal>qq[]</literal>,
<literal>qw[]</literal>). Backslashes must be escaped by doubling <literal>qw[]</literal>).
them.
</para> </para>
</tip> </tip>
<para>
The new function <function>empcomp</function> can be used like:
<programlisting>
SELECT name, empcomp(employee) FROM employee;
</programlisting>
</para>
<para> <para>
Here is an example of a function that will not work because file Here is an example of a function that will not work because file
system operations are not allowed for security reasons: system operations are not allowed for security reasons:
@ -174,9 +232,29 @@ CREATE FUNCTION badfunc() RETURNS integer AS '
The creation of the function will succeed, but executing it will not. The creation of the function will succeed, but executing it will not.
</para> </para>
<para> <para>
Note that if same function was created by superuser using language Note that if the same function was created by a superuser using language
<literal>plperlu</>, execution would succeed. <literal>plperlu</>, execution would succeed.
</para> </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> <para>
Access to the database itself from your Perl function can be done via Access to the database itself from your Perl function can be done via
an experimental module <ulink an experimental module <ulink
@ -188,8 +266,55 @@ CREATE FUNCTION badfunc() RETURNS integer AS '
with normal <acronym>DBI</> syntax. with normal <acronym>DBI</> syntax.
</para> </para>
</sect1> <para>
</chapter> 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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