Function to determine next payment date

I'm trying to create a function to determine the next due payment date
of a recurring expense.

I have a table containing the date the payment first started, and a
payment frequency key which relates to a payment ID in another table
containing a string defining how frequent the payment exists.

-- Cut down version of table definitions:

CREATE TABLE "tblRecuringExpenses"
(
   expense_id integer NOT NULL DEFAULT nextval('seq_expense_id'::regclass),
   expense_frequency character varying(1) NOT NULL,
   expense_startdate timestamp with time zone NOT NULL,
   CONSTRAINT pk_tblrecuringexpenses_expense_id PRIMARY KEY (expense_id),
   CONSTRAINT fk_tblrecuringexpenses_expense_frequency FOREIGN KEY
(expense_frequency)
       REFERENCES "tblRecuringFrequency" (frequency_id) MATCH SIMPLE
       ON UPDATE NO ACTION ON DELETE NO ACTION
)
WITHOUT OIDS;

CREATE TABLE "tblRecuringFrequency"
(
   frequency_id character varying(1) NOT NULL,
   next_payment character varying(40),
   CONSTRAINT pk_tblrecuringfrequency_frequncy_id PRIMARY KEY (frequency_id)
)
WITHOUT OIDS;


I figure the best way to get the next payment date is to create a
function that takes in the payment ID, then loops through adding the
payment frequency interval onto the start date until it gets to a date
that's greater than or equal to the current date.

Firstly... is this assumption correct or is there an easier way to do it?

Secondly... assuming my initial assumption is correct I've created the
following function in PL/pgSQL, however when I execute it, it runs for
minutes - where the two tables above I'm working with are dummy test
tables that contain 2 rows each and the calculations are simple enough
that I would assume a speedy conclusion - it would seem to me that my
loop is endless.

An explain analyze does me no good - that get's stuck in the same
endless loop, and a simple explain only gives me:
  "Result  (cost=0.00..0.01 rows=1 width=0)"


CREATE OR REPLACE FUNCTION fnNextPaymentDue(integer) RETURNS timestamp AS $$
DECLARE
    start_date timestamp;
    next_payment interval;
    loop_count int;
BEGIN
    SELECT INTO start_date (SELECT expense_startdate FROM
       "tblRecuringExpenses" WHERE expense_id=$1);
    SELECT INTO next_payment (select next_payment from
       "tblRecuringFrequency" JOIN "tblRecuringExpenses" on
       ("tblRecuringFrequency".frequency_id
        ="tblRecuringExpenses".expense_frequency)
       WHERE expense_id=$1)::interval;
LOOP
    if start_date >=current_date then
       exit;
    end if;
    start_date:=start_date+next_payment;
END LOOP;
RETURN start_date;
END;
$$ LANGUAGE PLPGSQL STABLE;

Since it's my first function in PL/pgSQL I've probably missed something
pretty obvious - but if someone could point it out to me that would be
muchly appreciated.

--
Paul Lambert
Database Administrator
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