PL/Perl Perl PL/Perl is a loadable procedural language that enables the Perl programming language to be used to write PostgreSQL functions. Normally, PL/Perl is installed as a trusted programming language named 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, require, and use (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 PostgreSQL user ID, as a C function can do. Thus, any unprivileged database user may be permitted to use this language. 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 untrusted language (usually named 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. If the --with-perl option was supplied to the configure configure script, the PostgreSQL build process will attempt to build the PL/Perl shared library and install it in the PostgreSQL library directory. On most platforms, since PL/Perl is a shared library, the libperl libperl 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: *** 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. If you see this, you will have to re-build and install Perl manually to be able to build PL/Perl. During the configuration process for Perl, request a shared library. After having reinstalled Perl, change to the directory src/pl/plperl in the PostgreSQL source tree and issue the commands gmake clean gmake all gmake install to complete the build and installation of the PL/Perl shared library. To install PL/Perl and/or PL/PerlU in a particular database, use the createlang script, for example createlang plperl dbname or createlang plperlu dbname. If a language is installed into template1, all subsequently created databases will have the language installed automatically. To create a function in the PL/Perl language, use the standard syntax CREATE FUNCTION funcname (argument-types) RETURNS return-type AS ' # PL/Perl function body ' LANGUAGE plperl; PL/PerlU is the same, except that the language should be specified as plperlu. The body of the function is ordinary Perl code. Arguments and results are handled as in any other Perl subroutine: arguments are passed in @_, and a result value is returned with 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: CREATE FUNCTION perl_max (integer, integer) RETURNS integer AS ' if ($_[0] > $_[1]) { return $_[0]; } return $_[1]; ' LANGUAGE plperl; If a NULL is passed to a function, the argument value will appear as undefined in Perl. The above function definition will not behave very nicely with NULL inputs (in fact, it will act as though they are zeroes). We could add WITH (isStrict) to the function definition to make PostgreSQL do something more reasonable: if a NULL is passed, the function will not be called at all, but will just return a NULL result automatically. Alternatively, we could check for undefined inputs in the function body. For example, suppose that we wanted perl_max with one null and one non-null argument to return the non-null argument, rather than NULL: CREATE FUNCTION perl_max (integer, integer) RETURNS integer AS ' my ($a,$b) = @_; if (! defined $a) { if (! defined $b) { return undef; } return $b; } if (! defined $b) { return $a; } if ($a > $b) { return $a; } return $b; ' LANGUAGE plperl; As shown above, to return a NULL from a PL/Perl function, return an undefined value. This can be done whether the function is strict or not. Composite-type arguments are passed to the function as references to hashes. The keys of the hash are the attribute names of the composite type. Here is an example: CREATE TABLE employee ( name text, basesalary integer, bonus integer ); CREATE FUNCTION empcomp(employee) RETURNS integer AS ' my ($emp) = @_; return $emp->{''basesalary''} + $emp->{''bonus''}; ' LANGUAGE plperl; SELECT name, empcomp(employee) FROM employee; There is not currently any support for returning a composite-type result value. Because the function body is passed as an SQL string literal to CREATE FUNCTION, 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 (q[], qq[], qw[]). Here is an example of a function that will not work because file system operations are not allowed for security reasons: CREATE FUNCTION badfunc() RETURNS integer AS ' open(TEMP, ">/tmp/badfile"); print TEMP "Gotcha!\n"; return 1; ' LANGUAGE plperl; The creation of the function will succeed, but executing it will not. Note that if the same function was created by a superuser using language plperlu, execution would succeed. 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 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. Access to the database itself from your Perl function can be done via an experimental module DBD::PgSPI (also available at CPAN mirror sites). This module makes available a DBI-compliant database-handle named $pg_dbh that can be used to perform queries with normal DBI syntax. PL/Perl itself presently provides only one additional Perl command: elog elog level, msg Emit a log or error message. Possible levels are DEBUG, NOTICE, and ERROR. DEBUG and NOTICE simply emit the given message into the postmaster log (and send it to the client too, in the case of NOTICE). ERROR raises an error condition: further execution of the function is abandoned, and the current transaction is aborted. 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. PL/Perl cannot currently be used to write trigger functions. DBD::PgSPI or similar capability should be integrated into the standard PostgreSQL distribution.