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The PostgreSQL 9.0 Reference Manual - Volume 2 - Programming Guide
by The PostgreSQL Global Development Group
Paperback (6"x9"), 478 pages
ISBN 9781906966065
RRP £14.95 ($19.95)

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11.3.1 Database Access from PL/Perl

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

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)
ISBN 9781906966065The PostgreSQL 9.0 Reference Manual - Volume 2 - Programming GuideSee the print edition