I need to "renumber" records in a table according to some "grouping"
fields, and I though of doing it with this stored procedure:

----------
CREATE PROCEDURE RENUM (
SIFKOR VARCHAR(10),
GOD INTEGER,
ORGJ SMALLINT,
START INTEGER)
AS
DECLARE VARIABLE D DATE;
DECLARE VARIABLE N INTEGER;
begin
select gen_id(tmp$gen, 0) from rdb$database into :n;
for
select date
from tkm
where sifkor=:sifkor and god=:god and orgj=:orgj
order by date
into :d
do
update tkm
set rbtk=gen_id(tmp$gen, 1)-:n+:start
where sifkor=:sifkor and god=:god
and orgj=:orgj and date=:d;
suspend;
end

--------------
The goal is to "group" records by (sifkor,god,orgj), and in each
group renumber the records (field "rbtk") ordered by date and
starting at :start (so the earliest record gets rbtk=:start,
the next one :start+1, etc.). The above procedure renumbers one
group.

I have two problems with the above code:
a) It works slowly (despite having index on (sifkor,god,orgj,date);
I think I should use cursors to do the updating, but are they
allowed inside a procedure? If true, can somebody write an
example of how they're used?)

b) (much more serious) It doesn't work :( Some numbers in the sequence
are strangely missing. For example, first few records in a group
get numbered correctly, then there's a gap of about 2-5 numbers,
then a sequence, etc. It's important that the numbers are in
sequential order, and I don't see any errors in my procedure :(