postgresql/doc/src/sgml/plperl.sgml

<!-- $PostgreSQL: pgsql/doc/src/sgml/plperl.sgml,v 2.86 2010/07/08 21:35:33 petere Exp $ -->

 <chapter id="plperl">
  <title>PL/Perl - Perl Procedural Language</title>

  <indexterm zone="plperl">
   <primary>PL/Perl</primary>
  </indexterm>

  <indexterm zone="plperl">
   <primary>Perl</primary>
  </indexterm>

  <para>
   PL/Perl is a loadable procedural language that enables you to write
   <productname>PostgreSQL</productname> functions in the
   <ulink url="http://www.perl.org">Perl programming language</ulink>.
  </para>

  <para>
   The main advantage to using PL/Perl is that this allows use,
   within stored functions, of the manyfold <quote>string
   munging</quote> operators and functions available for Perl.  Parsing
   complex strings might be easier using Perl than it is with the
   string functions and control structures provided in PL/pgSQL.
  </para>

  <para>
   To install PL/Perl in a particular database, use
   <literal>createlang plperl <replaceable>dbname</></literal>.
  </para>

  <tip>
   <para>
    If a language is installed into <literal>template1</>, all subsequently
    created databases will have the language installed automatically.
   </para>
  </tip>

  <note>
   <para>
    Users of source packages must specially enable the build of
    PL/Perl during the installation process.  (Refer to <xref
    linkend="install-short"> for more information.)  Users of
    binary packages might find PL/Perl in a separate subpackage.
   </para>
  </note>

 <sect1 id="plperl-funcs">
  <title>PL/Perl Functions and Arguments</title>

  <para>
   To create a function in the PL/Perl language, use the standard
   <xref linkend="sql-createfunction">
   syntax:

<programlisting>
CREATE FUNCTION <replaceable>funcname</replaceable> (<replaceable>argument-types</replaceable>) RETURNS <replaceable>return-type</replaceable> AS $$
    # PL/Perl function body
$$ LANGUAGE plperl;
</programlisting>

   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.
  </para>

  <para>
   PL/Perl also supports anonymous code blocks called with the
   <xref linkend="sql-do"> statement:

<programlisting>
DO $$
    # PL/Perl code
$$ LANGUAGE plperl;
</programlisting>

   An anonymous code block receives no arguments, and whatever value it
   might return is discarded.  Otherwise it behaves just like a function.
  </para>

  <note>
   <para>
    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 <literal>Variable "%s" will not stay shared</literal> and
    <literal>Variable "%s" is not available</literal> in the
    <citerefentry><refentrytitle>perldiag</></citerefentry> man page, or
    search the Internet for <quote>perl nested named subroutine</>.
   </para>
  </note>

  <para>
   The syntax of the <command>CREATE FUNCTION</command> command requires
   the function body to be written as a string constant.  It is usually
   most convenient to use dollar quoting (see <xref
   linkend="sql-syntax-dollar-quoting">) for the string constant.
   If you choose to use escape string syntax <literal>E''</>,
   you must double the single quote marks (<literal>'</>) and backslashes
   (<literal>\</>) used in the body of the function
   (see <xref linkend="sql-syntax-strings">).
  </para>

  <para>
   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.
  </para>

  <para>
   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 $$
    if ($_[0] &gt; $_[1]) { return $_[0]; }
    return $_[1];
$$ LANGUAGE plperl;
</programlisting>
  </para>

  <para>
   If an SQL null value<indexterm><primary>null value</><secondary
   sortas="PL/Perl">in PL/Perl</></indexterm> 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>STRICT</> to the function definition to make
   <productname>PostgreSQL</productname> 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 <function>perl_max</function> with
   one null and one nonnull argument to return the nonnull argument,
   rather than a null value:

<programlisting>
CREATE FUNCTION perl_max (integer, integer) RETURNS integer AS $$
    my ($x, $y) = @_;
    if (not defined $x) {
        return undef if not defined $y;
        return $y;
    }
    return $x if not defined $y;
    return $x if $x &gt; $y;
    return $y;
$$ LANGUAGE plperl;
</programlisting>
   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.
  </para>

  <para>
   Anything in a function argument that is not a reference is
   a string, which is in the standard <productname>PostgreSQL</productname>
   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 <function>decode_bytea</function>
   function can be used to convert an argument of
   type <type>bytea</> into unescaped binary.
  </para>

  <para>
   Similarly, values passed back to <productname>PostgreSQL</productname>
   must be in the external text representation format. For example, the
   <function>encode_bytea</function> function can be used to
   escape binary data for a return value of type <type>bytea</>.
  </para>

  <para>
   Perl can return <productname>PostgreSQL</productname> arrays as
   references to Perl arrays.  Here is an example:

<programlisting>
CREATE OR REPLACE function returns_array()
RETURNS text[][] AS $$
    return [['a&quot;b','c,d'],['e\\f','g']];
$$ LANGUAGE plperl;

select returns_array();
</programlisting>
  </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 (
    name text,
    basesalary integer,
    bonus integer
);

CREATE FUNCTION empcomp(employee) RETURNS integer AS $$
    my ($emp) = @_;
    return $emp-&gt;{basesalary} + $emp-&gt;{bonus};
$$ LANGUAGE plperl;

SELECT name, empcomp(employee.*) FROM employee;
</programlisting>
  </para>

  <para>
   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:

<programlisting>
CREATE TYPE testrowperl AS (f1 integer, f2 text, f3 text);

CREATE OR REPLACE FUNCTION perl_row() RETURNS testrowperl AS $$
    return {f2 =&gt; 'hello', f1 =&gt; 1, f3 =&gt; 'world'};
$$ LANGUAGE plperl;

SELECT * FROM perl_row();
</programlisting>

   Any columns in the declared result data type that are not present in the
   hash will be returned as null values.
  </para>

  <para>
    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
    <function>return_next</function> as illustrated below.  Note that
    after the last <function>return_next</function>, you must put
    either <literal>return</literal> or (better) <literal>return
    undef</literal>.

<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 $$
    return_next({ f1 =&gt; 1, f2 =&gt; 'Hello', f3 =&gt; 'World' });
    return_next({ f1 =&gt; 2, f2 =&gt; 'Hello', f3 =&gt; 'PostgreSQL' });
    return_next({ f1 =&gt; 3, f2 =&gt; 'Hello', f3 =&gt; 'PL/Perl' });
    return undef;
$$ LANGUAGE plperl;
</programlisting>

    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:

<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 $$
    return [
        { f1 =&gt; 1, f2 =&gt; 'Hello', f3 =&gt; 'World' },
        { f1 =&gt; 2, f2 =&gt; 'Hello', f3 =&gt; 'PostgreSQL' },
        { f1 =&gt; 3, f2 =&gt; 'Hello', f3 =&gt; 'PL/Perl' }
    ];
$$ LANGUAGE plperl;

SELECT * FROM perl_set();
</programlisting>
  </para>

  <para>
   If you wish to use the <literal>strict</> pragma with your code you 
   have a few options. For temporary global use you can <command>SET</> 
   <literal>plperl.use_strict</literal> to true.
   This will affect subsequent compilations of <application>PL/Perl</>
   functions, but not functions already compiled in the current session.
   For permanent global use you can set <literal>plperl.use_strict</literal>
   to true in the <filename>postgresql.conf</filename> file.
  </para>

  <para>
   For permanent use in specific functions you can simply put:
<programlisting>
use strict;
</programlisting>
   at the top of the function body.
  </para>

  <para>
  The <literal>feature</> pragma is also available to <function>use</> if your Perl is version 5.10.0 or higher.
  </para>

 </sect1>

 <sect1 id="plperl-data">
  <title>Data Values in PL/Perl</title>

  <para>
   The argument values supplied to a PL/Perl function's code are
   simply the input arguments converted to text form (just as if they
   had been displayed by a <command>SELECT</command> statement).
   Conversely, the <function>return</function> and <function>return_next</function>
   commands will accept any string that is acceptable input format
   for the function's declared return type.
  </para>
 </sect1>

 <sect1 id="plperl-builtins">
  <title>Built-in Functions</title>

 <sect2 id="plperl-database">
  <title>Database Access from PL/Perl</title>

  <para>
   Access to the database itself from your Perl function can be done
   via the following functions:
  </para>

   <variablelist>
    <varlistentry>
     <indexterm>
      <primary>spi_exec_query</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>
     <indexterm>
      <primary>spi_query</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>
     <indexterm>
      <primary>spi_fetchrow</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>
     <indexterm>
      <primary>spi_prepare</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>
     <indexterm>
      <primary>spi_exec_prepared</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>
     <indexterm>
      <primary>spi_query_prepared</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>
     <indexterm>
      <primary>spi_cursor_close</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>
     <indexterm>
      <primary>spi_freeplan</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>

     <term><literal><function>spi_exec_query</>(<replaceable>query</replaceable> [, <replaceable>max-rows</replaceable>])</literal></term>
     <listitem>
      <para>
       <literal>spi_exec_query</literal> executes an SQL command and
returns the entire row set as a reference to an array of hash
references.  <emphasis>You should only use this command when you know
that the result set will be relatively small.</emphasis>  Here is an
example of a query (<command>SELECT</command> command) with the
optional maximum number of rows:

<programlisting>
$rv = spi_exec_query('SELECT * FROM my_table', 5);
</programlisting>
        This returns up to 5 rows from the table
        <literal>my_table</literal>.  If <literal>my_table</literal>
        has a column <literal>my_column</literal>, you can get that
        value from row <literal>$i</literal> of the result like this:
<programlisting>
$foo = $rv-&gt;{rows}[$i]-&gt;{my_column};
</programlisting>
       The total number of rows returned from a <command>SELECT</command>
       query can be accessed like this:
<programlisting>
$nrows = $rv-&gt;{processed}
</programlisting>
      </para>

      <para>
       Here is an example using a different command type:
<programlisting>
$query = "INSERT INTO my_table VALUES (1, 'test')";
$rv = spi_exec_query($query);
</programlisting>
       You can then access the command status (e.g.,
       <literal>SPI_OK_INSERT</literal>) like this:
<programlisting>
$res = $rv-&gt;{status};
</programlisting>
       To get the number of rows affected, do:
<programlisting>
$nrows = $rv-&gt;{processed};
</programlisting>
      </para>

      <para>
       Here is a complete example:
<programlisting>
CREATE TABLE test (
    i int,
    v varchar
);

INSERT INTO test (i, v) VALUES (1, 'first line');
INSERT INTO test (i, v) VALUES (2, 'second line');
INSERT INTO test (i, v) VALUES (3, 'third line');
INSERT INTO test (i, v) VALUES (4, 'immortal');

CREATE OR REPLACE FUNCTION test_munge() RETURNS SETOF test AS $$
    my $rv = spi_exec_query('select i, v from test;');
    my $status = $rv-&gt;{status};
    my $nrows = $rv-&gt;{processed};
    foreach my $rn (0 .. $nrows - 1) {
        my $row = $rv-&gt;{rows}[$rn];
        $row-&gt;{i} += 200 if defined($row-&gt;{i});
        $row-&gt;{v} =~ tr/A-Za-z/a-zA-Z/ if (defined($row-&gt;{v}));
        return_next($row);
    }
    return undef;
$$ LANGUAGE plperl;

SELECT * FROM test_munge();
</programlisting>
    </para>
    </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal><function>spi_query</>(<replaceable>command</replaceable>)</literal></term>
     <term><literal><function>spi_fetchrow</>(<replaceable>cursor</replaceable>)</literal></term>
     <term><literal><function>spi_cursor_close</>(<replaceable>cursor</replaceable>)</literal></term>

    <listitem>
    <para>
    <literal>spi_query</literal> and <literal>spi_fetchrow</literal>
    work together as a pair for row sets which might be large, or for cases
    where you wish to return rows as they arrive.
    <literal>spi_fetchrow</literal> works <emphasis>only</emphasis> with
    <literal>spi_query</literal>. The following example illustrates how
    you use them together:

<programlisting>
CREATE TYPE foo_type AS (the_num INTEGER, the_text TEXT);

CREATE OR REPLACE FUNCTION lotsa_md5 (INTEGER) RETURNS SETOF foo_type AS $$
    use Digest::MD5 qw(md5_hex);
    my $file = '/usr/share/dict/words';
    my $t = localtime;
    elog(NOTICE, "opening file $file at $t" );
    open my $fh, '&lt;', $file # ooh, it's a file access!
        or elog(ERROR, "cannot open $file for reading: $!");
    my @words = &lt;$fh&gt;;
    close $fh;
    $t = localtime;
    elog(NOTICE, "closed file $file at $t");
    chomp(@words);
    my $row;
    my $sth = spi_query("SELECT * FROM generate_series(1,$_[0]) AS b(a)");
    while (defined ($row = spi_fetchrow($sth))) {
        return_next({
            the_num =&gt; $row-&gt;{a},
            the_text =&gt; md5_hex($words[rand @words])
        });
    }
    return;
$$ LANGUAGE plperlu;

SELECT * from lotsa_md5(500);
</programlisting>
    </para>

    <para>
     Normally, <function>spi_fetchrow</> should be repeated until it
     returns <literal>undef</literal>, indicating that there are no more
     rows to read.  The cursor returned by <literal>spi_query</literal>
     is automatically freed when
     <function>spi_fetchrow</> returns <literal>undef</literal>.
     If you do not wish to read all the rows, instead call
     <function>spi_cursor_close</> to free the cursor.
     Failure to do so will result in memory leaks.
    </para>

    </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal><function>spi_prepare</>(<replaceable>command</replaceable>, <replaceable>argument types</replaceable>)</literal></term>
     <term><literal><function>spi_query_prepared</>(<replaceable>plan</replaceable>, <replaceable>arguments</replaceable>)</literal></term>
     <term><literal><function>spi_exec_prepared</>(<replaceable>plan</replaceable> [, <replaceable>attributes</replaceable>], <replaceable>arguments</replaceable>)</literal></term>
     <term><literal><function>spi_freeplan</>(<replaceable>plan</replaceable>)</literal></term>

    <listitem>
    <para>
    <literal>spi_prepare</literal>, <literal>spi_query_prepared</literal>, <literal>spi_exec_prepared</literal>,
    and <literal>spi_freeplan</literal> implement the same functionality but for prepared queries.
    <literal>spi_prepare</literal> accepts a query string with numbered argument placeholders ($1, $2, etc)
    and a string list of argument types:
<programlisting>
$plan = spi_prepare('SELECT * FROM test WHERE id &gt; $1 AND name = $2',
                                                     'INTEGER', 'TEXT');
</programlisting>
    Once a query plan is prepared by a call to <literal>spi_prepare</literal>, the plan can be used instead
    of the string query, either in <literal>spi_exec_prepared</literal>, where the result is the same as returned
    by <literal>spi_exec_query</literal>, or in <literal>spi_query_prepared</literal> which returns a cursor
    exactly as <literal>spi_query</literal> does, which can be later passed to <literal>spi_fetchrow</literal>.
    The optional second parameter to <literal>spi_exec_prepared</literal> is a hash reference of attributes;
    the only attribute currently supported is <literal>limit</literal>, which sets the maximum number of rows returned by a query. 
    </para>

    <para>
    The advantage of prepared queries is that is it possible to use one prepared plan for more
    than one query execution. After the plan is not needed anymore, it can be freed with
    <literal>spi_freeplan</literal>:
    </para>

    <para>
    <programlisting>
CREATE OR REPLACE FUNCTION init() RETURNS VOID AS $$
        $_SHARED{my_plan} = spi_prepare('SELECT (now() + $1)::date AS now',
                                        'INTERVAL');
$$ LANGUAGE plperl;

CREATE OR REPLACE FUNCTION add_time( INTERVAL ) RETURNS TEXT AS $$
        return spi_exec_prepared(
                $_SHARED{my_plan},
                $_[0]
        )->{rows}->[0]->{now};
$$ LANGUAGE plperl;

CREATE OR REPLACE FUNCTION done() RETURNS VOID AS $$
        spi_freeplan( $_SHARED{my_plan});
        undef $_SHARED{my_plan};
$$ LANGUAGE plperl;

SELECT init();
SELECT add_time('1 day'), add_time('2 days'), add_time('3 days');
SELECT done();

  add_time  |  add_time  |  add_time
------------+------------+------------
 2005-12-10 | 2005-12-11 | 2005-12-12
    </programlisting>
    </para>

    <para>
    Note that the parameter subscript in <literal>spi_prepare</literal> is defined via
    $1, $2, $3, etc, so avoid declaring query strings in double quotes that might easily
    lead to hard-to-catch bugs.
    </para>

    <para>
    Another example illustrates usage of an optional parameter in <literal>spi_exec_prepared</literal>:
    </para>

    <para>
    <programlisting>
CREATE TABLE hosts AS SELECT id, ('192.168.1.'||id)::inet AS address
                      FROM generate_series(1,3) AS id;

CREATE OR REPLACE FUNCTION init_hosts_query() RETURNS VOID AS $$
        $_SHARED{plan} = spi_prepare('SELECT * FROM hosts 
                                      WHERE address &lt;&lt; $1', 'inet');
$$ LANGUAGE plperl;

CREATE OR REPLACE FUNCTION query_hosts(inet) RETURNS SETOF hosts AS $$
        return spi_exec_prepared(
                $_SHARED{plan},
                {limit =&gt; 2},
                $_[0]
        )->{rows};
$$ LANGUAGE plperl;

CREATE OR REPLACE FUNCTION release_hosts_query() RETURNS VOID AS $$
        spi_freeplan($_SHARED{plan});
        undef $_SHARED{plan};
$$ LANGUAGE plperl;

SELECT init_hosts_query();
SELECT query_hosts('192.168.1.0/30');
SELECT release_hosts_query();

    query_hosts    
-----------------
 (1,192.168.1.1)
 (2,192.168.1.2)
(2 rows)
    </programlisting>
    </para>
    </listitem>
    </varlistentry>
   </variablelist>
 </sect2>

 <sect2 id="plperl-utility-functions">
  <title>Utility functions in PL/Perl</title>

   <variablelist>
    <varlistentry>
     <indexterm>
      <primary>elog</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>

     <term><literal><function>elog</>(<replaceable>level</replaceable>, <replaceable>msg</replaceable>)</literal></term>
     <listitem>
      <para>
       Emit a log or error message. Possible levels are
       <literal>DEBUG</>, <literal>LOG</>, <literal>INFO</>,
       <literal>NOTICE</>, <literal>WARNING</>, and <literal>ERROR</>.
       <literal>ERROR</>
        raises an error condition; if this is not trapped by the surrounding
        Perl code, the error propagates out to the calling query, causing
        the current transaction or subtransaction to be aborted.  This
        is effectively the same as the Perl <literal>die</> command.
        The other levels only generate messages of different
        priority levels.
        Whether messages of a particular priority are reported to the client,
        written to the server log, or both is controlled by the
        <xref linkend="guc-log-min-messages"> and
        <xref linkend="guc-client-min-messages"> configuration
        variables. See <xref linkend="runtime-config"> for more
        information.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <indexterm>
      <primary>quote_literal</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>

     <term><literal><function>quote_literal</>(<replaceable>string</replaceable>)</literal></term>
     <listitem>
      <para>
        Return the given string suitably quoted to be used as a string literal in an SQL
        statement string. Embedded single-quotes and backslashes are properly doubled.
        Note that <function>quote_literal</> returns undef on undef input; if the argument
        might be undef, <function>quote_nullable</> is often more suitable.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <indexterm>
      <primary>quote_nullable</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>

     <term><literal><function>quote_nullable</>(<replaceable>string</replaceable>)</literal></term>
     <listitem>
      <para>
        Return the given string suitably quoted to be used as a string literal in an SQL
        statement string; or, if the argument is undef, return the unquoted string "NULL".
        Embedded single-quotes and backslashes are properly doubled.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <indexterm>
      <primary>quote_ident</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>

     <term><literal><function>quote_ident</>(<replaceable>string</replaceable>)</literal></term>
     <listitem>
      <para>
        Return the given string suitably quoted to be used as an identifier in
        an SQL statement string. Quotes are added only if necessary (i.e., if
        the string contains non-identifier characters or would be case-folded).
        Embedded quotes are properly doubled.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <indexterm>
      <primary>decode_bytea</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>

     <term><literal><function>decode_bytea</>(<replaceable>string</replaceable>)</literal></term>
     <listitem>
      <para>
        Return the unescaped binary data represented by the contents of the given string,
        which should be bytea encoded.
        </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <indexterm>
      <primary>encode_bytea</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>

     <term><literal><function>encode_bytea</>(<replaceable>string</replaceable>)</literal></term>
     <listitem>
      <para>
        Return the bytea encoded form of the binary data contents of the given string.
        </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <indexterm>
      <primary>encode_array_literal</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>

     <term><literal><function>encode_array_literal</>(<replaceable>array</replaceable>)</literal></term>
     <term><literal><function>encode_array_literal</>(<replaceable>array</replaceable>, <replaceable>delimiter</replaceable>)</literal></term>
     <listitem>
      <para>
        Returns the contents of the referenced array as a string in array literal format
        (see <xref linkend="arrays-input">).
        Returns the argument value unaltered if it's not a reference to an array.
        The delimiter used between elements of the array literal defaults to "<literal>, </literal>"
        if a delimiter is not specified or is undef.
        </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <indexterm>
      <primary>encode_array_constructor</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>

     <term><literal><function>encode_array_constructor</>(<replaceable>array</replaceable>)</literal></term>
     <listitem>
      <para>
        Returns the contents of the referenced array as a string in array constructor format
        (see <xref linkend="sql-syntax-array-constructors">).
        Individual values are quoted using <function>quote_nullable</function>.
        Returns the argument value, quoted using <function>quote_nullable</function>,
        if it's not a reference to an array.
        </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <indexterm>
      <primary>looks_like_number</primary>
      <secondary>in PL/Perl</secondary>
     </indexterm>

     <term><literal><function>looks_like_number</>(<replaceable>string</replaceable>)</literal></term>
     <listitem>
      <para>
        Returns a true value if the content of the given string looks like a
        number, according to Perl, returns false otherwise.
        Returns undef if the argument is undef.  Leading and trailing space is
        ignored. <literal>Inf</> and <literal>Infinity</> are regarded as numbers.
        </para>
     </listitem>
    </varlistentry>

   </variablelist>
 </sect2>
 </sect1>

 <sect1 id="plperl-global">
  <title>Global Values in PL/Perl</title>

  <para>
    You can use the global hash <varname>%_SHARED</varname> to store
    data, including code references, between function calls for the
    lifetime of the current session.
  </para>

  <para>
    Here is a simple example for shared data:
<programlisting>
CREATE OR REPLACE FUNCTION set_var(name text, val text) RETURNS text AS $$
    if ($_SHARED{$_[0]} = $_[1]) {
        return 'ok';
    } else {
        return "cannot set shared variable $_[0] to $_[1]";
    }
$$ LANGUAGE plperl;

CREATE OR REPLACE FUNCTION get_var(name text) RETURNS text AS $$
    return $_SHARED{$_[0]};
$$ LANGUAGE plperl;

SELECT set_var('sample', 'Hello, PL/Perl!  How''s tricks?');
SELECT get_var('sample');
</programlisting>
  </para>

  <para>
   Here is a slightly more complicated example using a code reference:

<programlisting>
CREATE OR REPLACE FUNCTION myfuncs() RETURNS void AS $$
    $_SHARED{myquote} = sub {
        my $arg = shift;
        $arg =~ s/(['\\])/\\$1/g;
        return "'$arg'";
    };
$$ LANGUAGE plperl;

SELECT myfuncs(); /* initializes the function */

/* Set up a function that uses the quote function */

CREATE OR REPLACE FUNCTION use_quote(TEXT) RETURNS text AS $$
    my $text_to_quote = shift;
    my $qfunc = $_SHARED{myquote};
    return &amp;$qfunc($text_to_quote);
$$ LANGUAGE plperl;
</programlisting>

   (You could have replaced the above with the one-liner
   <literal>return $_SHARED{myquote}-&gt;($_[0]);</literal>
   at the expense of readability.)
  </para>

  <para>
    The <varname>%_SHARED</varname> variable and other global state within
    the language are public data, available to all PL/Perl functions within a
    session. Use with care, especially in situations that involve use of 
    multiple roles or <literal>SECURITY DEFINER</> functions.
  </para>
 </sect1>

 <sect1 id="plperl-trusted">
  <title>Trusted and Untrusted PL/Perl</title>

  <indexterm zone="plperl-trusted">
   <primary>trusted</primary>
   <secondary>PL/Perl</secondary>
  </indexterm>

  <para>
   Normally, PL/Perl is installed as a <quote>trusted</> programming
   language named <literal>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,
   <literal>require</literal>, and <literal>use</literal> (for
   external modules).  There is no way to access internals of the
   database server process or to gain OS-level access with the
   permissions of the server process,
   as a C function can do.  Thus, any unprivileged database user can
   be permitted to use this language.
  </para>

  <para>
   Here is an example of a function that will not work because file
   system operations are not allowed for security reasons:
<programlisting>
CREATE FUNCTION badfunc() RETURNS integer AS $$
    my $tmpfile = "/tmp/badfile";
    open my $fh, '&gt;', $tmpfile
        or elog(ERROR, qq{could not open the file "$tmpfile": $!});
    print $fh "Testing writing to a file\n";
    close $fh or elog(ERROR, qq{could not close the file "$tmpfile": $!});
    return 1;
$$ LANGUAGE plperl;
</programlisting>
    The creation of this function will fail as its use of a forbidden
    operation will be caught by the validator.
  </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 called
   <application>PL/PerlU</application><indexterm><primary>PL/PerlU</></indexterm>).
   In this case the full Perl language is available.  If the
   <command>createlang</command> program is used to install the
   language, the language name <literal>plperlu</literal> will select
   the untrusted PL/Perl variant.
  </para>

  <para>
   The writer of a <application>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>

  <para>
   If the above function was created by a superuser using the language
   <literal>plperlu</>, execution would succeed.
  </para>

  <para>
   In the same way, anonymous code blocks written in Perl can use
   restricted operations if the language is specified as
   <literal>plperlu</> rather than <literal>plperl</>, but the caller
   must be a superuser.
  </para>

  <note>
    <para>
      For security reasons, to stop a leak of privileged operations from
      <application>PL/PerlU</> to <application>PL/Perl</>, these two languages
      have to run in separate instances of the Perl interpreter. If your
      Perl installation has been appropriately compiled, this is not a problem.
      However, not all installations are compiled with the requisite flags.
      If <productname>PostgreSQL</> detects that this is the case then it will
      not start a second interpreter, but instead create an error. In
      consequence, in such an installation, you cannot use both
      <application>PL/PerlU</> and <application>PL/Perl</> in the same backend
      process. The remedy for this is to obtain a Perl installation configured
      with the appropriate flags, namely either <literal>usemultiplicity</>
      or <literal>useithreads</>. <literal>usemultiplicity</> is preferred
      unless you actually need to use threads. For more details, see the
      <citerefentry><refentrytitle>perlembed</></citerefentry> man page.
    </para>
  </note>

 </sect1>

 <sect1 id="plperl-triggers">
  <title>PL/Perl Triggers</title>

  <para>
   PL/Perl can be used to write trigger functions.  In a trigger function,
   the hash reference <varname>$_TD</varname> contains information about the
   current trigger event. <varname>$_TD</> is a global variable,
   which gets a separate local value for each invocation of the trigger.
   The fields of the <varname>$_TD</varname> hash reference are:

   <variablelist>
    <varlistentry>
     <term><literal>$_TD-&gt;{new}{foo}</literal></term>
     <listitem>
      <para>
       <literal>NEW</literal> value of column <literal>foo</literal>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{old}{foo}</literal></term>
     <listitem>
      <para>
       <literal>OLD</literal> value of column <literal>foo</literal>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{name}</literal></term>
     <listitem>
      <para>
       Name of the trigger being called
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{event}</literal></term>
     <listitem>
      <para>
       Trigger event: <literal>INSERT</>, <literal>UPDATE</>,
       <literal>DELETE</>, <literal>TRUNCATE</>, or <literal>UNKNOWN</>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{when}</literal></term>
     <listitem>
      <para>
       When the trigger was called: <literal>BEFORE</literal>, <literal>AFTER</literal>, or <literal>UNKNOWN</literal>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{level}</literal></term>
     <listitem>
      <para>
       The trigger level: <literal>ROW</literal>, <literal>STATEMENT</literal>, or <literal>UNKNOWN</literal>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{relid}</literal></term>
     <listitem>
      <para>
       OID of the table on which the trigger fired
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{table_name}</literal></term>
     <listitem>
      <para>
       Name of the table on which the trigger fired
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{relname}</literal></term>
     <listitem>
      <para>
       Name of the table on which the trigger fired. This has been deprecated,
       and could be removed in a future release.
       Please use $_TD-&gt;{table_name} instead.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{table_schema}</literal></term>
     <listitem>
      <para>
       Name of the schema in which the table on which the trigger fired, is
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>$_TD-&gt;{argc}</literal></term>
     <listitem>
      <para>
       Number of arguments of the trigger function
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>@{$_TD-&gt;{args}}</literal></term>
     <listitem>
      <para>
       Arguments of the trigger function.  Does not exist if <literal>$_TD-&gt;{argc}</literal> is 0.
      </para>
     </listitem>
    </varlistentry>

   </variablelist>
  </para>

  <para>
   Row-level triggers can return one of the following:

   <variablelist>
    <varlistentry>
     <term><literal>return;</literal></term>
     <listitem>
      <para>
       Execute the operation
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>"SKIP"</literal></term>
     <listitem>
      <para>
       Don't execute the operation
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>"MODIFY"</literal></term>
     <listitem>
      <para>
       Indicates that the <literal>NEW</literal> row was modified by
       the trigger function
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   Here is an example of a trigger function, illustrating some of the
   above:
<programlisting>
CREATE TABLE test (
    i int,
    v varchar
);

CREATE OR REPLACE FUNCTION valid_id() RETURNS trigger AS $$
    if (($_TD-&gt;{new}{i} &gt;= 100) || ($_TD-&gt;{new}{i} &lt;= 0)) {
        return "SKIP";    # skip INSERT/UPDATE command
    } elsif ($_TD-&gt;{new}{v} ne "immortal") {
        $_TD-&gt;{new}{v} .= "(modified by trigger)";
        return "MODIFY";  # modify row and execute INSERT/UPDATE command
    } else {
        return;           # execute INSERT/UPDATE command
    }
$$ LANGUAGE plperl;

CREATE TRIGGER test_valid_id_trig
    BEFORE INSERT OR UPDATE ON test
    FOR EACH ROW EXECUTE PROCEDURE valid_id();
</programlisting>
  </para>
 </sect1>

 <sect1 id="plperl-under-the-hood">
  <title>PL/Perl Under the Hood</title>

 <sect2 id="plperl-config">
  <title>Configuration</title>

  <para>
  This section lists configuration parameters that affect <application>PL/Perl</>.
  To set any of these parameters before <application>PL/Perl</> has been loaded,
  it is necessary to have added <quote><literal>plperl</></> to the
  <xref linkend="guc-custom-variable-classes"> list in
  <filename>postgresql.conf</filename>.
  </para>

  <variablelist>

     <varlistentry id="guc-plperl-on-init" xreflabel="plperl.on_init">
      <term><varname>plperl.on_init</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>plperl.on_init</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
       Specifies Perl code to be executed when a Perl interpreter is first initialized
       and before it is specialized for use by <literal>plperl</> or <literal>plperlu</>.
       The SPI functions are not available when this code is executed.
       If the code fails with an error it will abort the initialization of the interpreter
       and propagate out to the calling query, causing the current transaction
       or subtransaction to be aborted.
       </para>
       <para>
       The Perl code is limited to a single string. Longer code can be placed
       into a module and loaded by the <literal>on_init</> string.
       Examples:
<programlisting>
plperl.on_init = 'require "plperlinit.pl"'
plperl.on_init = 'use lib "/my/app"; use MyApp::PgInit;'
</programlisting>
       </para>
       <para>
       Any modules loaded by <literal>plperl.on_init</>, either directly or
       indirectly, will be available for use by <literal>plperl</>.  This may
       create a security risk. To see what modules have been loaded you can use:
<programlisting>
DO 'elog(WARNING, join ", ", sort keys %INC)' language plperl;
</programlisting>
       </para>
       <para>
       Initialization will happen in the postmaster if the plperl library is included
       in <literal>shared_preload_libraries</> (see <xref linkend="guc-shared-preload-libraries">),
       in which case extra consideration should be given to the risk of destabilizing the postmaster.
       </para>
       <para>
       This parameter can only be set in the postgresql.conf file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-plperl-on-plperl-init" xreflabel="plperl.on_plperl_init">
      <term><varname>plperl.on_plperl_init</varname> (<type>string</type>)</term>
      <term><varname>plperl.on_plperlu_init</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>plperl.on_plperl_init</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>plperl.on_plperlu_init</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
       These parameters specify Perl code to be executed when the
       <literal>plperl</>, or <literal>plperlu</> language is first used in a
       session.  Changes to these parameters after the corresponding language
       has been used will have no effect.
       The SPI functions are not available when this code is executed.
       Only superusers can change these settings.
       The Perl code in <literal>plperl.on_plperl_init</> can only perform trusted operations.
       </para>
       <para>
       The effect of setting these parameters is very similar to executing a
       <literal>DO</> command with the Perl code before any other use of the
       language.  The parameters are useful when you want to execute the Perl
       code automatically on every connection, or when a connection is not
       interactive. The parameters can be used by non-superusers by having a
       superuser execute an <literal>ALTER USER ... SET ...</> command.
       For example:
<programlisting>
ALTER USER joe SET plperl.on_plperl_init = '$_SHARED{debug} = 1';
</programlisting>
       </para>
       <para>
       If the code fails with an error it will abort the initialization and
       propagate out to the calling query, causing the current transaction or
       subtransaction to be aborted. Any changes within Perl won't be undone.
       If the language is used again the initialization will be repeated.
       </para>
       <para>
       The difference between these two settings and the 
       <literal>plperl.on_init</> setting is that these can be used for
       settings specific to the trusted or untrusted language variant, such
       as setting values in the <varname>%_SHARED</> variable. By contrast,
       <literal>plperl.on_init</> is more useful for doing things like
       setting the library search path for <productname>Perl</> or 
       loading Perl modules that don't interact directly with 
       <productname>PostgreSQL</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-plperl-use-strict" xreflabel="plperl.use_strict">
      <term><varname>plperl.use_strict</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>plperl.use_strict</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
       When set true subsequent compilations of PL/Perl functions have the <literal>strict</> pragma enabled.
       This parameter does not affect functions already compiled in the current session.
       </para>
      </listitem>
     </varlistentry>

  </variablelist>
</sect2>

 <sect2 id="plperl-missing">
  <title>Limitations and Missing Features</title>

  <para>
   The following features are currently missing from PL/Perl, but they
   would make welcome contributions.

   <itemizedlist>
    <listitem>
     <para>
      PL/Perl functions cannot call each other directly.
     </para>
    </listitem>

    <listitem>
     <para>
      SPI is not yet fully implemented.
     </para>
    </listitem>

    <listitem>
     <para>
      If you are fetching very large data sets using
      <literal>spi_exec_query</literal>, you should be aware that
      these will all go into memory.  You can avoid this by using
      <literal>spi_query</literal>/<literal>spi_fetchrow</literal> as
      illustrated earlier.
     </para>
     <para>
        A similar problem occurs if a set-returning function passes a
        large set of rows back to PostgreSQL via <literal>return</literal>. You
        can avoid this problem too by instead using
        <literal>return_next</literal> for each row returned, as shown
        previously.
     </para>
    </listitem>

     <listitem>
      <para>
        When a session ends normally, not due to a fatal error, any
        <literal>END</> blocks that have been defined are executed.
        Currently no other actions are performed. Specifically, 
        file handles are not automatically flushed and objects are 
        not automatically destroyed.
      </para>
     </listitem>
   </itemizedlist>
  </para>
 </sect2>

 </sect1>

</chapter>