Why this is slow ?

[hello world]

when i use this function in a loop for 111 times , the machine seems like hanging. why ? there is index for all tables. any better idea in performance.

Create function f_Payment (v_Agent varchar(3), v_Comp varchar(100) ) returns varchar(100)
BEGIN

declare a1 varchar(20);
declare a2 varchar(20);
declare a3 varchar(20);

declare v_Result varchar(100);

select sum(amount) into a1 from reload where agentid=v_Agent and compid=v_Comp;
select sum(amount) into a2 from payment where agentid=v_Agent and compid=v_Comp;
select sum(callcost) into a3 from cdr where agentid=v_Agent and compid=v_Comp;

IF(a1 IS NULL) THEN
select 0 into a1;
select concat(a1, "," , a2, "," , a3) into v_Result;
ELSE
select concat(a1, "," , a2, "," , a3) into v_Result;
END IF;

IF(a2 IS NULL) THEN
select 0 into a2;
select concat(a1, "," , a2, "," , a3) into v_Result;
ELSE
select concat(a1, "," , a2, "," , a3) into v_Result;
END IF;

IF(a3 IS NULL) THEN
select 0 into a3;
select concat(a1, "," , a2, "," , a3) into v_Result;
ELSE
select concat(a1, "," , a2, "," , a3) into v_Result;
END IF;


return v_Result;

END;