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 <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.com">Perl</ulink> programming language.
  </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="installation"> 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 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.
  </para>

   <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 regular single-quoted string constant syntax,
    you must escape single quote marks (<literal>'</>) and backslashes
    (<literal>\</>) used in the body of the function, typically by
    doubling them (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 $$
    my ($a,$b) = @_;
    if (! defined $a) {
        if (! defined $b) { return undef; }
        return $b;
    }
    if (! defined $b) { return $a; }
    if ($a &gt; $b) { return $a; }
    return $b;
$$ 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>
   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 NULLs.
  </para>

  <para>
   PL/Perl functions can also return sets of either scalar or composite
   types.  To do this, return a reference to an array that contains
   either scalars or references to hashes, respectively.  Here are
   some simple examples:

<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>

   Note that when you do this, Perl will have to build the entire array in
   memory; therefore the technique does not scale to very large result sets.
  </para>

    <para>
     <application>PL/Perl</> does not currently have full support for
     domain types: it treats a domain the same as the underlying scalar
     type.  This means that constraints associated with the domain will
     not be enforced.  This is not an issue for function arguments, but
     it is a hazard if you declare a <application>PL/Perl</> function
     as returning a domain type.
    </para>
 </sect1>

 <sect1 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 function <function>spi_exec_query</function> described
   below, or via an experimental module <ulink
   url="http://www.cpan.org/modules/by-module/DBD/APILOS/"><literal>DBD::PgSPI</literal></ulink>
   (also available at <ulink
   url="http://www.cpan.org/SITES.html"><acronym>CPAN</> mirror
   sites</ulink>).  This module makes available a
   <acronym>DBI</>-compliant database-handle named
   <varname>$pg_dbh</varname> that can be used to perform queries with
   normal <acronym>DBI</>
   syntax.<indexterm><primary>DBI</></indexterm>
  </para>

  <para>
   PL/Perl itself presently provides two additional Perl commands:

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

     <term><literal><function>spi_exec_query</>(<replaceable>query</replaceable> [, <replaceable>max-rows</replaceable>])</literal></term>
     <term><literal><function>spi_exec_query</>(<replaceable>command</replaceable>)</literal></term>
     <listitem>
      <para>
       Executes an SQL command.  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 FUNCTION test_munge() RETURNS SETOF test AS $$
    my $res = [];
    my $rv = spi_exec_query('select i, v from test;');
    my $status = $rv-&gt;{status};
    my $nrows = $rv-&gt;{processed};
    foreach my $rn (0 .. $nrows - 1) {
        my $row = $rv-&gt;{rows}[$rn];
        $row-&gt;{i} += 200 if defined($row-&gt;{i});
        $row-&gt;{v} =~ tr/A-Za-z/a-zA-Z/ if (defined($row-&gt;{v}));
        push @$res, $row;
    }
    return $res;
$$ LANGUAGE plperl;

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

    <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>
   </variablelist>
  </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 <literal>return</> command will accept any string
   that is acceptable input format for the function's declared return
   type.  So, within the PL/Perl function,
   all values are just text strings.
  </para>
 </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 $$
    if ($_SHARED{$_[0]} = $_[1]) {
        return 'ok';
    } else {
        return "can't set shared variable $_[0] to $_[1]";
    }
$$ 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>
   For security reasons, PL/Perl executes functions called by any one SQL role
   in a separate Perl interpreter for that role.  This prevents accidental or
   malicious interference by one user with the behavior of another user's
   PL/Perl functions.  Each such interpreter has its own value of the
   <varname>%_SHARED</varname> variable and other global state.  Thus, two
   PL/Perl functions will share the same value of <varname>%_SHARED</varname>
   if and only if they are executed by the same SQL role.  In an application
   wherein a single session executes code under multiple SQL roles (via
   <literal>SECURITY DEFINER</> functions, use of <command>SET ROLE</>, etc)
   you may need to take explicit steps to ensure that PL/Perl functions can
   share data via <varname>%_SHARED</varname>.  To do that, make sure that
   functions that should communicate are owned by the same user, and mark
   them <literal>SECURITY DEFINER</>.  You must of course take care that
   such functions can't be used to do anything unintended.
  </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 may
   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 $$
    open(TEMP, "&gt;/tmp/badfile");
    print TEMP "Gotcha!\n";
    return 1;
$$ LANGUAGE plperl;
</programlisting>
   The creation of the function will succeed, but executing it will not.
  </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>

  <note>
   <para>
    While <application>PL/Perl</> functions run in a separate Perl
    interpreter for each SQL role, all <application>PL/PerlU</> functions
    executed in a given session run in a single Perl interpreter (which is
    not any of the ones used for <application>PL/Perl</> functions).
    This allows <application>PL/PerlU</> functions to share data freely,
    but no communication can occur between <application>PL/Perl</> and
    <application>PL/PerlU</> functions.
   </para>
  </note>

  <note>
   <para>
    Perl cannot support multiple interpreters within one process unless
    it was built 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.)
    If <application>PL/Perl</> is used with a copy of Perl that was not built
    this way, then it is only possible to have one Perl interpreter per
    session, and so any one session can only execute either
    <application>PL/PerlU</> functions, or <application>PL/Perl</> functions
    that are all called by the same SQL role.
   </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>$_TD</varname> contains information about the
   current trigger event.  The fields of the <varname>$_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</>, 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;{relname}</literal></term>
     <listitem>
      <para>
       Name of the table on which the trigger fired
      </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 $_TD-&gt;{argc} is 0.
      </para>
     </listitem>
    </varlistentry>

   </variablelist>
  </para>

  <para>
   Triggers can return one of the following:

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

    <varlistentry>
     <term><literal>"SKIP"</literal></term>
     <listitem>
      <para>
       Don't execute the statement
      </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-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 (because they
      are anonymous subroutines inside Perl).
     </para>
    </listitem>

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

    <listitem>
     <para>
      In the current implementation, if you are fetching or returning
      very large data sets, you should be aware that these will all go
      into memory.
     </para>
    </listitem>
   </itemizedlist>
  </para>
 </sect1>

</chapter>

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