sqlは複数の行の最も早い日付を選択します

select customer_name,
       kwh,
       reading_date, 
       reading_time
from (
   select customer_name,
          kwh,
          reading_time,
          reading_date,
          row_number() over (partition by customer_name order by reading_time) as rn
   from readings
   where reading_date = date '2012-11-17'
) t
where rn = 1

別の方法として：

select r1.customer_name,
       r1.kwh, 
       r1.reading_date,
       r1.reading_time
from readings r1
where reading_date = date '2012-11-17'
and reading_time = (select min(r2.reading_time)
                    from readings
                    where r2.customer_name = r1.customer_name
                    and r2.read_date = r1.reading_date);

しかし、私は最初のものがより速いことを期待します。

ところで：なぜ日付と時刻を2つの別々の列に保存するのですか？これはtimestampでより適切に処理できることをご存知ですか 列？