To create a function in the PL/Perl language, use the standard CREATE FUNCTION syntax:
CREATE FUNCTIONfuncname
(argument-types
) RETURNSreturn-type
-- function attributes can go here AS $$ # PL/Perl function body goes here $$ LANGUAGE plperl;
The body of the function is ordinary Perl code. In fact, the PL/Perl glue code wraps it inside a Perl subroutine. A PL/Perl function is called in a scalar context, so it can't return a list. You can return non-scalar values (arrays, records, and sets) by returning a reference, as discussed below.
In a PL/Perl procedure, any return value from the Perl code is ignored.
PL/Perl also supports anonymous code blocks called with the DO statement:
DO $$ # PL/Perl code $$ LANGUAGE plperl;
An anonymous code block receives no arguments, and whatever value it might return is discarded. Otherwise it behaves just like a function.
The use of named nested subroutines is dangerous in Perl, especially if
they refer to lexical variables in the enclosing scope. Because a PL/Perl
function is wrapped in a subroutine, any named subroutine you place inside
one will be nested. In general, it is far safer to create anonymous
subroutines which you call via a coderef. For more information, see the
entries for Variable "%s" will not stay shared
and
Variable "%s" is not available
in the
perldiag man page, or
search the Internet for “perl nested named subroutine”.
The syntax of the CREATE FUNCTION
command requires
the function body to be written as a string constant. It is usually
most convenient to use dollar quoting (see 4.1.2.4절) for the string constant.
If you choose to use escape string syntax E''
,
you must double any single quote marks ('
) and backslashes
(\
) used in the body of the function
(see 4.1.2.1절).
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;
Arguments will be converted from the database's encoding to UTF-8 for use inside PL/Perl, and then converted from UTF-8 back to the database encoding upon return.
If an SQL null value 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 STRICT
to the function definition to make
PostgreSQL do something more reasonable:
if a null value 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 nonnull argument to return the nonnull argument,
rather than a null value:
CREATE FUNCTION perl_max (integer, integer) RETURNS integer AS $$ my ($x, $y) = @_; if (not defined $x) { return undef if not defined $y; return $y; } return $x if not defined $y; return $x if $x > $y; return $y; $$ LANGUAGE plperl;
As shown above, to return an SQL null value from a PL/Perl function, return an undefined value. This can be done whether the function is strict or not.
Anything in a function argument that is not a reference is
a string, which is in the standard PostgreSQL
external text representation for the relevant data type. In the case of
ordinary numeric or text types, Perl will just do the right thing and
the programmer will normally not have to worry about it. However, in
other cases the argument will need to be converted into a form that is
more usable in Perl. For example, the decode_bytea
function can be used to convert an argument of
type bytea
into unescaped binary.
Similarly, values passed back to PostgreSQL
must be in the external text representation format. For example, the
encode_bytea
function can be used to
escape binary data for a return value of type bytea
.
One case that is particularly important is boolean values. As just
stated, the default behavior for bool
values is that they
are passed to Perl as text, thus either 't'
or 'f'
. This is problematic, since Perl will not
treat 'f'
as false! It is possible to improve matters
by using a “transform” (see
CREATE TRANSFORM). Suitable transforms are provided
by the bool_plperl
extension. To use it, install
the extension:
CREATE EXTENSION bool_plperl; -- or bool_plperlu for PL/PerlU
Then use the TRANSFORM
function attribute for a
PL/Perl function that takes or returns bool
, for example:
CREATE FUNCTION perl_and(bool, bool) RETURNS bool TRANSFORM FOR TYPE bool AS $$ my ($a, $b) = @_; return $a && $b; $$ LANGUAGE plperl;
When this transform is applied, bool
arguments will be seen
by Perl as being 1
or empty, thus properly true or
false. If the function result is type bool
, it will be true
or false according to whether Perl would evaluate the returned value as
true.
Similar transformations are also performed for boolean query arguments
and results of SPI queries performed inside the function
(44.3.1절).
Perl can return PostgreSQL arrays as references to Perl arrays. Here is an example:
CREATE OR REPLACE function returns_array() RETURNS text[][] AS $$ return [['a"b','c,d'],['e\\f','g']]; $$ LANGUAGE plperl; select returns_array();
Perl passes PostgreSQL arrays as a blessed
PostgreSQL::InServer::ARRAY
object. This object may be treated as an array
reference or a string, allowing for backward compatibility with Perl
code written for PostgreSQL versions below 9.1 to
run. For example:
CREATE OR REPLACE FUNCTION concat_array_elements(text[]) RETURNS TEXT AS $$ my $arg = shift; my $result = ""; return undef if (!defined $arg); # as an array reference for (@$arg) { $result .= $_; } # also works as a string $result .= $arg; return $result; $$ LANGUAGE plperl; SELECT concat_array_elements(ARRAY['PL','/','Perl']);
Multidimensional arrays are represented as references to lower-dimensional arrays of references in a way common to every Perl programmer.
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;
A PL/Perl function can return a composite-type result using the same approach: return a reference to a hash that has the required attributes. For example:
CREATE TYPE testrowperl AS (f1 integer, f2 text, f3 text); CREATE OR REPLACE FUNCTION perl_row() RETURNS testrowperl AS $$ return {f2 => 'hello', f1 => 1, f3 => 'world'}; $$ LANGUAGE plperl; SELECT * FROM perl_row();
Any columns in the declared result data type that are not present in the hash will be returned as null values.
Similarly, output arguments of procedures can be returned as a hash reference:
CREATE PROCEDURE perl_triple(INOUT a integer, INOUT b integer) AS $$ my ($a, $b) = @_; return {a => $a * 3, b => $b * 3}; $$ LANGUAGE plperl; CALL perl_triple(5, 10);
PL/Perl functions can also return sets of either scalar or
composite types. Usually you'll want to return rows one at a
time, both to speed up startup time and to keep from queuing up
the entire result set in memory. You can do this with
return_next
as illustrated below. Note that
after the last return_next
, you must put
either return
or (better) return
undef
.
CREATE OR REPLACE FUNCTION perl_set_int(int) RETURNS SETOF INTEGER AS $$ foreach (0..$_[0]) { return_next($_); } return undef; $$ LANGUAGE plperl; SELECT * FROM perl_set_int(5); CREATE OR REPLACE FUNCTION perl_set() RETURNS SETOF testrowperl AS $$ return_next({ f1 => 1, f2 => 'Hello', f3 => 'World' }); return_next({ f1 => 2, f2 => 'Hello', f3 => 'PostgreSQL' }); return_next({ f1 => 3, f2 => 'Hello', f3 => 'PL/Perl' }); return undef; $$ LANGUAGE plperl;
For small result sets, you can return a reference to an array that contains either scalars, references to arrays, or references to hashes for simple types, array types, and composite types, respectively. Here are some simple examples of returning the entire result set as an array reference:
CREATE OR REPLACE FUNCTION perl_set_int(int) RETURNS SETOF INTEGER AS $$ return [0..$_[0]]; $$ LANGUAGE plperl; SELECT * FROM perl_set_int(5); CREATE OR REPLACE FUNCTION perl_set() RETURNS SETOF testrowperl AS $$ return [ { f1 => 1, f2 => 'Hello', f3 => 'World' }, { f1 => 2, f2 => 'Hello', f3 => 'PostgreSQL' }, { f1 => 3, f2 => 'Hello', f3 => 'PL/Perl' } ]; $$ LANGUAGE plperl; SELECT * FROM perl_set();
If you wish to use the strict
pragma with your code you
have a few options. For temporary global use you can SET
plperl.use_strict
to true.
This will affect subsequent compilations of PL/Perl
functions, but not functions already compiled in the current session.
For permanent global use you can set plperl.use_strict
to true in the postgresql.conf
file.
For permanent use in specific functions you can simply put:
use strict;
at the top of the function body.
The feature
pragma is also available to use
if your Perl is version 5.10.0 or higher.