performance issues when using functional index

Hi ,
i am from telecom industry and my working version is oracle 10g standard edition in RAC

i have requirment where i have to dynamically allocate a number from the pool table

the below is my pool table structure

create table xyz
(
  POOL_NUMBER      VARCHAR2(20) primary key,
  POOL_ID          NUMBER(5),
  ALLOCATED_NUMBER VARCHAR2(20),
  ALLOCATED_TIME   DATE,
  INSTANCE_ID      NUMBER(2)
)
/

 the below query is used to dynamically allocate a number from pool table

 SELECT pool_number
                INTO v_pool_number
                FROM  xyz
               WHERE pool_id = 1
                     AND decode(allocated_time, NULL, 1, NULL) = 1
                     AND rownum = 1;

i have created the functional index for the column allocated_time as it queries in the above