I have to write pl/pgsql function that has to be triggered not before/after insert, but in definite time. Does postgresql allows to do this ?
there is no 'cron' built into postgresql. you could write your function, then have an external cron job invoke it, like: psql -c "select yourfunction()"
Note also that if you are planning some development with the coming release 9.3, you could also achieve that inside the server by using a custom bgworker.