求和SQL优化

SELECT id
      ,currencyCode
      ,(CASE WHEN total<=3 THEN amount ELSE (3 - last_total)/conversion_rate END) AS target_amount
      ,(CASE WHEN total<=3 THEN target_amount ELSE 3 - last_total END) AS target_amount
  FROM (
SELECT t.*
      ,t.amount*r.conversion_rate as target_amount
      ,SUM(t.amount*r.conversion_rate) OVER(ORDER BY t.amount*r.conversion_rate,id) as total
      ,SUM(t.amount*r.conversion_rate) OVER(ORDER BY t.amount*r.conversion_rate,id) - t.amount*r.conversion_rate as last_total
      ,r.conversion_rate
  FROM t_money t, t_rates r
WHERE t.currencyCode = r.from_currency
       AND r.to_currency = 'usd'
)
WHERE total<=3
      OR total>3
         AND last_total<3
ORDER BY total;


--------------
SELECT id,
       currencyCode,
       (CASE
         WHEN total <= 3 THEN
          amount
         ELSE
          (3 - last_total)/nvl((select nvl(conversion_rate, 1)
                                           from t_rates
                                          where from_currency = currencycode
                                            and upper(to_currency) = 'USD'),
                                         1)
       END)amount,
       (CASE
         WHEN total <= 3 THEN
          target_amount
         ELSE
          3 - last_total
       END) AS target_amount
  FROM (SELECT t.*,
               t.amount * r.conversion_rate as target_amount,
               SUM(t.amount * r.conversion_rate) OVER(ORDER BY t.amount * r.conversion_rate, id) as total,
               SUM(t.amount * r.conversion_rate) OVER(ORDER BY t.amount * r.conversion_rate, id) - t.amount * r.conversion_rate as last_total
          FROM t_money t, t_rates r
         WHERE t.currencyCode = r.from_currency
           AND r.to_currency = 'usd')
WHERE total <= 3
    OR total > 3
   AND last_total < 3
ORDER BY total;