PL/Perl Functions and Arguments

PostgreSQL 8.3.3 Documentation
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# 40.1. PL/Perl Functions and Arguments

To create a function in the PL/Perl language, use the standard CREATE FUNCTION syntax:

    CREATE FUNCTION funcname (argument-types) RETURNS return-type AS $$
        # PL/Perl function body
    $$ 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 must always return a scalar value. You can return more complex structures (arrays, records, and sets) by returning a reference, as discussed below. Never return a list.

> Note: 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 create will be nested. In general, it is far safer to create anonymous subroutines which you call via a coderef. See the `perldiag` man page for more details.

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 Section 4.1.2.2) for the string constant. If you choose to use escape string syntax `E''`, you must double the single quote marks (`'`) and backslashes (`\`) used in the body of the function (see Section 4.1.2.1).

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 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 $$
        my ($x,$y) = @_;
        if (! defined $x) {
            if (! defined $y) { return undef; }
            return $y;
        }
        if (! defined $y) { return $x; }
        if ($x > $y) { return $x; }
        return $y;
    $$ 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, here is how to convert an argument of type `bytea` into unescaped binary data:

        my $arg = shift;
        $arg =~ s!\\(?:\\|(\d{3}))!$1 ? chr(oct($1)) : "\\"!ge;

Similarly, values passed back to PostgreSQL must be in the external text representation format. For example, here is how to escape binary data for a return value of type `bytea`:

        $retval =~ s!(\\|[^ -~])!sprintf("\\%03o",ord($1))!ge;
        return $retval;

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();

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.

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 queueing 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, the easiest way to do so is to `SET` `plperl.use_strict` to true. This parameter affects subsequent compilations of PL/Perl functions, but not functions already compiled in the current session. To set the parameter before PL/Perl has been loaded, it is necessary to have added "`plperl`" to the custom_variable_classes list in `postgresql.conf`.

Another way to use the `strict` pragma is to put:

    use strict;

in the function body. But this only works in PL/PerlU functions, since `use` is not a trusted operation. In PL/Perl functions you can instead do:

    BEGIN { strict->import(); }

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