Postgres function optimisation

I am running the below function. Testing it with a much smaller table works as expected (18 rows - ~400ms). However, when pointed at my real data (315000 rows), it is running 48hrs and still going. This is much longer than I expected under a linear extrapolation.

1. Is there a better way to do this?


2. Is there a way to test if it is doing what it should be doing while still running?

Is there any way to optimise the below function?

DO 
$do$ 
DECLARE r public.tablex%rowtype; 
BEGIN 
FOR r IN SELECT id FROM public.tablex 
LOOP 
IF (select cast((select trunc(random() * 6 + 1)) as integer) = 5) THEN 
UPDATE public.tablex SET test='variable1' WHERE id = r.id; 
ELSIF (select cast((select trunc(random() * 6 + 1)) as integer) = 6) THEN 
UPDATE public.tablex SET test='variable2' WHERE id = r.id; 
END IF; 
END LOOP; 
RETURN; 
END 
$do$;