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>Chapter 41. PL/Perl - Perl Procedural Language</TD
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><H1
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><A
NAME="PLPERL-FUNCS"
>41.1. PL/Perl Functions and Arguments</A
></H1
><P
> To create a function in the PL/Perl language, use the standard
<A
HREF="sql-createfunction.html"
>CREATE FUNCTION</A
>
syntax:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION <TT
CLASS="REPLACEABLE"
><I
>funcname</I
></TT
> (<TT
CLASS="REPLACEABLE"
><I
>argument-types</I
></TT
>) RETURNS <TT
CLASS="REPLACEABLE"
><I
>return-type</I
></TT
> AS $$
# PL/Perl function body
$$ LANGUAGE plperl;</PRE
><P>
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.
</P
><P
> PL/Perl also supports anonymous code blocks called with the
<A
HREF="sql-do.html"
>DO</A
> statement:
</P><PRE
CLASS="PROGRAMLISTING"
>DO $$
# PL/Perl code
$$ LANGUAGE plperl;</PRE
><P>
An anonymous code block receives no arguments, and whatever value it
might return is discarded. Otherwise it behaves just like a function.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> 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 <TT
CLASS="LITERAL"
>Variable "%s" will not stay shared</TT
> and
<TT
CLASS="LITERAL"
>Variable "%s" is not available</TT
> in the
<SPAN
CLASS="CITEREFENTRY"
><SPAN
CLASS="REFENTRYTITLE"
>perldiag</SPAN
></SPAN
> man page, or
search the Internet for <SPAN
CLASS="QUOTE"
>"perl nested named subroutine"</SPAN
>.
</P
></BLOCKQUOTE
></DIV
><P
> The syntax of the <TT
CLASS="COMMAND"
>CREATE FUNCTION</TT
> command requires
the function body to be written as a string constant. It is usually
most convenient to use dollar quoting (see <A
HREF="sql-syntax-lexical.html#SQL-SYNTAX-DOLLAR-QUOTING"
>Section 4.1.2.4</A
>) for the string constant.
If you choose to use escape string syntax <TT
CLASS="LITERAL"
>E''</TT
>,
you must double any single quote marks (<TT
CLASS="LITERAL"
>'</TT
>) and backslashes
(<TT
CLASS="LITERAL"
>\</TT
>) used in the body of the function
(see <A
HREF="sql-syntax-lexical.html#SQL-SYNTAX-STRINGS"
>Section 4.1.2.1</A
>).
</P
><P
> Arguments and results are handled as in any other Perl subroutine:
arguments are passed in <TT
CLASS="VARNAME"
>@_</TT
>, and a result value
is returned with <TT
CLASS="LITERAL"
>return</TT
> or as the last expression
evaluated in the function.
</P
><P
> For example, a function returning the greater of two integer values
could be defined as:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION perl_max (integer, integer) RETURNS integer AS $$
if ($_[0] > $_[1]) { return $_[0]; }
return $_[1];
$$ LANGUAGE plperl;</PRE
><P>
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> 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.
</P
></BLOCKQUOTE
></DIV
><P
> If an SQL null value is passed to a function,
the argument value will appear as <SPAN
CLASS="QUOTE"
>"undefined"</SPAN
> 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 <TT
CLASS="LITERAL"
>STRICT</TT
> to the function definition to make
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 <CODE
CLASS="FUNCTION"
>perl_max</CODE
> with
one null and one nonnull argument to return the nonnull argument,
rather than a null value:
</P><PRE
CLASS="PROGRAMLISTING"
>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;</PRE
><P>
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.
</P
><P
> Anything in a function argument that is not a reference is
a string, which is in the standard <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
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 <CODE
CLASS="FUNCTION"
>decode_bytea</CODE
>
function can be used to convert an argument of
type <TT
CLASS="TYPE"
>bytea</TT
> into unescaped binary.
</P
><P
> Similarly, values passed back to <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
must be in the external text representation format. For example, the
<CODE
CLASS="FUNCTION"
>encode_bytea</CODE
> function can be used to
escape binary data for a return value of type <TT
CLASS="TYPE"
>bytea</TT
>.
</P
><P
> Perl can return <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> arrays as
references to Perl arrays. Here is an example:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE OR REPLACE function returns_array()
RETURNS text[][] AS $$
return [['a"b','c,d'],['e\\f','g']];
$$ LANGUAGE plperl;
select returns_array();</PRE
><P>
</P
><P
> Perl passes <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> versions below 9.1 to
run. For example:
</P><PRE
CLASS="PROGRAMLISTING"
>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']);</PRE
><P>
</P><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> Multi-dimensional arrays are represented as references to
lower-dimensional arrays of references in a way common to every Perl
programmer.
</P
></BLOCKQUOTE
></DIV
><P>
</P
><P
> 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:
</P><PRE
CLASS="PROGRAMLISTING"
>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;</PRE
><P>
</P
><P
> 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:
</P><PRE
CLASS="PROGRAMLISTING"
>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();</PRE
><P>
Any columns in the declared result data type that are not present in the
hash will be returned as null values.
</P
><P
> 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
<CODE
CLASS="FUNCTION"
>return_next</CODE
> as illustrated below. Note that
after the last <CODE
CLASS="FUNCTION"
>return_next</CODE
>, you must put
either <TT
CLASS="LITERAL"
>return</TT
> or (better) <TT
CLASS="LITERAL"
>return
undef</TT
>.
</P><PRE
CLASS="PROGRAMLISTING"
>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;</PRE
><P>
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:
</P><PRE
CLASS="PROGRAMLISTING"
>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();</PRE
><P>
</P
><P
> If you wish to use the <TT
CLASS="LITERAL"
>strict</TT
> pragma with your code you
have a few options. For temporary global use you can <TT
CLASS="COMMAND"
>SET</TT
>
<TT
CLASS="LITERAL"
>plperl.use_strict</TT
> to true.
This will affect subsequent compilations of <SPAN
CLASS="APPLICATION"
>PL/Perl</SPAN
>
functions, but not functions already compiled in the current session.
For permanent global use you can set <TT
CLASS="LITERAL"
>plperl.use_strict</TT
>
to true in the <TT
CLASS="FILENAME"
>postgresql.conf</TT
> file.
</P
><P
> For permanent use in specific functions you can simply put:
</P><PRE
CLASS="PROGRAMLISTING"
>use strict;</PRE
><P>
at the top of the function body.
</P
><P
> The <TT
CLASS="LITERAL"
>feature</TT
> pragma is also available to <CODE
CLASS="FUNCTION"
>use</CODE
> if your Perl is version 5.10.0 or higher.
</P
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