create table t(
  id serial primary key,
  url text not null,
  param1 text,
  param2 text,
  result text
);

CREATE OR REPLACE FUNCTION http_req()
  RETURNS trigger AS
$BODY$
import urllib

params = {
  'param1': TD['new']['param1'],
  'param2': TD['new']['param2']
}

TD['new']['result'] = urllib.urlopen(TD['new']['url'] + '?' + urllib.urlencode(params)).read()
return  'MODIFY'
$BODY$
  LANGUAGE plpythonu;


CREATE TRIGGER ins_t
  BEFORE INSERT  ON t
  FOR EACH ROW EXECUTE PROCEDURE http_req();

#стандартные модули (из site-packages):
import sys, os, time

#модули из произвольных директорий:
sys.path.append('/opt/my_module/')
import my_module