runtime of the same query in function differs on 2 degree!

Hi folks.

What's wrong with planner that executes my query in function?:
(i mean no explanation but runtime)


tele=# EXPLAIN analyze select calc_total(6916799, 1062363600, 1064955599);
                                        QUERY PLAN
------------------------------------------------------------------------------------------
 Result  (cost=0.00..0.01 rows=1 width=0) (actual time=36919.37..36919.37 rows=1 loops=1)
 Total runtime: 36919.40 msec
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

tele=# \df+ calc_total
...
declare
        usr alias for $1;
        d1 alias for $2;
        d2   alias for $3;
        res integer;
begin
        select sum(cost) into res
                from bills where
                        (parent(user_id) = usr or user_id = usr)
                        and dat >= d1 and dat < d2;
        if res is not null then
                return res;
        else
                return 0;
        end if;
end;

tele=# EXPLAIN analyze select sum(cost) from bills where (parent(user_id) = 6916799 or user_id = 6916799) and dat >=
1062363600and dat < 10649555 
99;
                                                                             QUERY PLAN

--------------------------------------------------------------------------------------------------------------------------------------------------
------------------
 Aggregate  (cost=17902.80..17902.80 rows=1 width=4) (actual time=101.04..101.04 rows=1 loops=1)
   ->  Index Scan using bills_parent_user_id_idx, bills_userid_dat_idx on bills  (cost=0.00..17901.11 rows=679 width=4)
(actualtime=101.03..101.0 
3 rows=0 loops=1)
         Index Cond: ((parent(user_id) = 6916799) OR ((user_id = 6916799) AND (dat >= 1062363600) AND (dat <
1064955599)))
         Filter: (((parent(user_id) = 6916799) OR (user_id = 6916799)) AND (dat >= 1062363600) AND (dat < 1064955599))
 Total runtime: 101.14 msec
 ^^^^^^^^^^^^^^^^^^^^^^^^^^

So the query is the same as in calc_total(usr,d1,d2) function,
but execute time extremely differs.

Is it normal?

Thanks,
  Andriy Tkachuk.