PL/Perl Perl PL/Perl is a loadable procedural language that enables you to write PostgreSQL functions in the Perl programming language. The main advantage to using PL/Perl is that this allows use, within stored functions, of the manyfold string munging 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. To install PL/Perl in a particular database, use createlang plperl dbname. If a language is installed into template1, all subsequently created databases will have the language installed automatically. Users of source packages must specially enable the build of PL/Perl during the installation process. (Refer to for more information.) Users of binary packages might find PL/Perl in a separate subpackage. To create a function in the PL/Perl language, use the standard 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 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. PL/Perl also supports anonymous code blocks called with the 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 create will be nested. In general, it is far safer to create anonymous subroutines which you call via a coderef. See Variable "%s" will not stay shared and Variable "%s" is not available in 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 ) 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 ). 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 valuenull valuein PL/Perl 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 to escape binary data for a return value of type bytea. 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 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. 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 SELECT statement). Conversely, the return and return_next commands will accept any string that is acceptable input format for the function's declared return type. Access to the database itself from your Perl function can be done via the following functions: spi_exec_query in PL/Perl spi_query in PL/Perl spi_fetchrow in PL/Perl spi_prepare in PL/Perl spi_exec_prepared in PL/Perl spi_query_prepared in PL/Perl spi_cursor_close in PL/Perl spi_freeplan in PL/Perl spi_exec_query(query [, max-rows]) spi_exec_query executes an SQL command and returns the entire row set as a reference to an array of hash references. You should only use this command when you know that the result set will be relatively small. Here is an example of a query (SELECT command) with the optional maximum number of rows: $rv = spi_exec_query('SELECT * FROM my_table', 5); This returns up to 5 rows from the table my_table. If my_table has a column my_column, you can get that value from row $i of the result like this: $foo = $rv->{rows}[$i]->{my_column}; The total number of rows returned from a SELECT query can be accessed like this: $nrows = $rv->{processed} Here is an example using a different command type: $query = "INSERT INTO my_table VALUES (1, 'test')"; $rv = spi_exec_query($query); You can then access the command status (e.g., SPI_OK_INSERT) like this: $res = $rv->{status}; To get the number of rows affected, do: $nrows = $rv->{processed}; Here is a complete example: 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 $$ my $rv = spi_exec_query('select i, v from test;'); my $status = $rv->{status}; my $nrows = $rv->{processed}; foreach my $rn (0 .. $nrows - 1) { my $row = $rv->{rows}[$rn]; $row->{i} += 200 if defined($row->{i}); $row->{v} =~ tr/A-Za-z/a-zA-Z/ if (defined($row->{v})); return_next($row); } return undef; $$ LANGUAGE plperl; SELECT * FROM test_munge(); spi_query(command) spi_fetchrow(cursor) spi_cursor_close(cursor) spi_query and spi_fetchrow work together as a pair for row sets which might be large, or for cases where you wish to return rows as they arrive. spi_fetchrow works only with spi_query. The following example illustrates how you use them together: CREATE TYPE foo_type AS (the_num INTEGER, the_text TEXT); CREATE OR REPLACE FUNCTION lotsa_md5 (INTEGER) RETURNS SETOF foo_type AS $$ use Digest::MD5 qw(md5_hex); my $file = '/usr/share/dict/words'; my $t = localtime; elog(NOTICE, "opening file $file at $t" ); open my $fh, '<', $file # ooh, it's a file access! or elog(ERROR, "cannot open $file for reading: $!"); my @words = <$fh>; close $fh; $t = localtime; elog(NOTICE, "closed file $file at $t"); chomp(@words); my $row; my $sth = spi_query("SELECT * FROM generate_series(1,$_[0]) AS b(a)"); while (defined ($row = spi_fetchrow($sth))) { return_next({ the_num => $row->{a}, the_text => md5_hex($words[rand @words]) }); } return; $$ LANGUAGE plperlu; SELECT * from lotsa_md5(500); Normally, spi_fetchrow should be repeated until it returns undef, indicating that there are no more rows to read. The cursor returned by spi_query is automatically freed when spi_fetchrow returns undef. If you do not wish to read all the rows, instead call spi_cursor_close to free the cursor. Failure to do so will result in memory leaks. spi_prepare(command, argument types) spi_query_prepared(plan, arguments) spi_exec_prepared(plan [, attributes], arguments) spi_freeplan(plan) spi_prepare, spi_query_prepared, spi_exec_prepared, and spi_freeplan implement the same functionality but for prepared queries. spi_prepare accepts a query string with numbered argument placeholders ($1, $2, etc) and a string list of argument types: $plan = spi_prepare('SELECT * FROM test WHERE id > $1 AND name = $2', 'INTEGER', 'TEXT'); Once a query plan is prepared by a call to spi_prepare, the plan can be used instead of the string query, either in spi_exec_prepared, where the result is the same as returned by spi_exec_query, or in spi_query_prepared which returns a cursor exactly as spi_query does, which can be later passed to spi_fetchrow. The optional second parameter to spi_exec_prepared is a hash reference of attributes; the only attribute currently supported is limit, which sets the maximum number of rows returned by a query. 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 spi_freeplan: 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 Note that the parameter subscript in spi_prepare is defined via $1, $2, $3, etc, so avoid declaring query strings in double quotes that might easily lead to hard-to-catch bugs. Another example illustrates usage of an optional parameter in spi_exec_prepared: 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 $$ $_SHARED{plan} = spi_prepare('SELECT * FROM hosts WHERE address << $1', 'inet'); $$ LANGUAGE plperl; CREATE OR REPLACE FUNCTION query_hosts(inet) RETURNS SETOF hosts AS $$ return spi_exec_prepared( $_SHARED{plan}, {limit => 2}, $_[0] )->{rows}; $$ 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) elog in PL/Perl elog(level, msg) Emit a log or error message. Possible levels are DEBUG, LOG, INFO, NOTICE, WARNING, and ERROR. 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 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 and configuration variables. See for more information. quote_literal in PL/Perl quote_literal(string) 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 quote_literal returns undef on undef input; if the argument might be undef, quote_nullable is often more suitable. quote_nullable in PL/Perl quote_nullable(string) 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. quote_ident in PL/Perl quote_ident(string) 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. decode_bytea in PL/Perl decode_bytea(string) Return the unescaped binary data represented by the contents of the given string, which should be bytea encoded. encode_bytea in PL/Perl encode_bytea(string) Return the bytea encoded form of the binary data contents of the given string. encode_array_literal in PL/Perl encode_array_literal(array) encode_array_literal(array, delimiter) Returns the contents of the referenced array as a string in array literal format (see ). 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 ", " if a delimiter is not specified or is undef. encode_array_constructor in PL/Perl encode_array_constructor(array) Returns the contents of the referenced array as a string in array constructor format (see ). Individual values are quoted using quote_nullable. Returns the argument value, quoted using quote_nullable, if it's not a reference to an array. looks_like_number in PL/Perl looks_like_number(string) 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. Inf and Infinity are regarded as numbers. You can use the global hash %_SHARED to store data, including code references, between function calls for the lifetime of the current session. Here is a simple example for shared data: 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'); Here is a slightly more complicated example using a code reference: 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 $$ my $text_to_quote = shift; my $qfunc = $_SHARED{myquote}; return &$qfunc($text_to_quote); $$ LANGUAGE plperl; (You could have replaced the above with the one-liner return $_SHARED{myquote}->($_[0]); at the expense of readability.) trusted PL/Perl Normally, PL/Perl is installed as a trusted programming language named 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, require, and use (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. Here is an example of a function that will not work because file system operations are not allowed for security reasons: CREATE FUNCTION badfunc() RETURNS integer AS $$ my $tmpfile = "/tmp/badfile"; open my $fh, '>', $tmpfile or elog(ERROR, qq{could not open the file "$tmpfile": $!}); print $fh "Testing writing to a file\n"; close $fh or elog(ERROR, qq{could not close the file "$tmpfile": $!}); return 1; $$ LANGUAGE plperl; The creation of this function will fail as its use of a forbidden operation will be caught by the validator. 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 untrusted language (usually called PL/PerlUPL/PerlU). In this case the full Perl language is available. If the createlang program is used to install the language, the language name plperlu will select the untrusted PL/Perl variant. The writer of a 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. If the above function was created by a superuser using the language plperlu, execution would succeed. In the same way, anonymous code blocks written in Perl can use restricted operations if the language is specified as plperlu rather than plperl, but the caller must be a superuser. For security reasons, to stop a leak of privileged operations from PL/PerlU to 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 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 PL/PerlU and PL/Perl in the same backend process. The remedy for this is to obtain a Perl installation configured with the appropriate flags, namely either usemultiplicity or useithreads. usemultiplicity is preferred unless you actually need to use threads. For more details, see the perlembed man page. PL/Perl can be used to write trigger functions. In a trigger function, the hash reference $_TD contains information about the current trigger event. $_TD is a global variable, which gets a separate local value for each invocation of the trigger. The fields of the $_TD hash reference are: $_TD->{new}{foo} NEW value of column foo $_TD->{old}{foo} OLD value of column foo $_TD->{name} Name of the trigger being called $_TD->{event} Trigger event: INSERT, UPDATE, DELETE, TRUNCATE, or UNKNOWN $_TD->{when} When the trigger was called: BEFORE, AFTER, or UNKNOWN $_TD->{level} The trigger level: ROW, STATEMENT, or UNKNOWN $_TD->{relid} OID of the table on which the trigger fired $_TD->{table_name} Name of the table on which the trigger fired $_TD->{relname} Name of the table on which the trigger fired. This has been deprecated, and could be removed in a future release. Please use $_TD->{table_name} instead. $_TD->{table_schema} Name of the schema in which the table on which the trigger fired, is $_TD->{argc} Number of arguments of the trigger function @{$_TD->{args}} Arguments of the trigger function. Does not exist if $_TD->{argc} is 0. Row-level triggers can return one of the following: return; Execute the operation "SKIP" Don't execute the operation "MODIFY" Indicates that the NEW row was modified by the trigger function Here is an example of a trigger function, illustrating some of the above: CREATE TABLE test ( i int, v varchar ); CREATE OR REPLACE FUNCTION valid_id() RETURNS trigger AS $$ if (($_TD->{new}{i} >= 100) || ($_TD->{new}{i} <= 0)) { return "SKIP"; # skip INSERT/UPDATE command } elsif ($_TD->{new}{v} ne "immortal") { $_TD->{new}{v} .= "(modified by trigger)"; return "MODIFY"; # modify row and execute INSERT/UPDATE command } else { return; # execute INSERT/UPDATE command } $$ LANGUAGE plperl; CREATE TRIGGER test_valid_id_trig BEFORE INSERT OR UPDATE ON test FOR EACH ROW EXECUTE PROCEDURE valid_id(); This section lists configuration parameters that affect PL/Perl. To set any of these parameters before PL/Perl has been loaded, it is necessary to have added plperl to the list in postgresql.conf. plperl.on_perl_init (string) plperl.on_perl_init configuration parameter Specifies perl code to be executed when a perl interpreter is first initialized. 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. The perl code is limited to a single string. Longer code can be placed into a module and loaded by the on_perl_init string. Examples: plplerl.on_perl_init = '$ENV{NYTPROF}="start=no"; require Devel::NYTProf::PgPLPerl' plplerl.on_perl_init = 'use lib "/my/app"; use MyApp::PgInit;' Initialization will happen in the postmaster if the plperl library is included in shared_preload_libraries (see ), in which case extra consideration should be given to the risk of destabilizing the postmaster. This parameter can only be set in the postgresql.conf file or on the server command line. plperl.use_strict (boolean) plperl.use_strict configuration parameter When set true subsequent compilations of PL/Perl functions have the strict pragma enabled. This parameter does not affect functions already compiled in the current session. The following features are currently missing from PL/Perl, but they would make welcome contributions. PL/Perl functions cannot call each other directly. SPI is not yet fully implemented. If you are fetching very large data sets using spi_exec_query, you should be aware that these will all go into memory. You can avoid this by using spi_query/spi_fetchrow as illustrated earlier. A similar problem occurs if a set-returning function passes a large set of rows back to PostgreSQL via return. You can avoid this problem too by instead using return_next for each row returned, as shown previously. When a session ends normally, not due to a fatal error, any 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.