pgrouting is very slow

I have the following code but the output is empty. It is very slow. I want a pair to pair pgr_dijstracost rather than many to many. Is there a feasible solution?

DO
$do$
DECLARE
   ii int := 1;
BEGIN
   WHILE ii < 1396381
   LOOP
     INSERT INTO public.loop (start_vid,end_vid,agg_cost)
     Select * from pgr_dijkstraCost(
    'select gid as id, source, target, length as cost from table1'
    ,(select table1.source from table1 Inner join table2 on table2.segment = table1.segment where table2.id = ii)
    ,(select table1.target from table1Inner join table2 on table2.segment2= table1.segment where table2.id = ii)
    ,false);
    ii := ii + 1;
   END LOOP;
END
$do$