请教一条oracle语句优化

select v.ic_seq,

        v.ve_name,

        v.ic_code,

        v.deploy_type,

        v.deploy_no_in,

        v.deploy_no_out,

        v.deploy_type_out,

        v.zone_code_1,

        v.zone_code_2,

        v.customs_code,

        v.region_code,

        (select max(pass_time) as pass_time

           from sen_car_pass s

          where s.ic_seq = v.ic_seq

            and s.zone_code = v.zone_code_1

            and s.io_flag = 'O'    group by s.ic_seq) as O,

        (select max(pass_time)

           from sen_car_pass a

          where a.ic_seq = v.ic_seq

            and a.zone_code = v.zone_code_2

            and a.io_flag = 'I'    group by a.ic_seq) as I

   from veh_ic_issue v

  where  bus_type = '6'