排序会减慢速度，这是肯定的。
您的第一个查询看起来不错;涉及到多少行？你期望什么样的优化？我猜你关心的是执行时间;如果是，查询需要多长时间？检查了解释计划吗？
同时，试试

WITH
   temp
   AS
      (SELECT nam,
              rson,
              url,
              ROW_NUMBER () OVER (ORDER BY d_timestamp DESC) rn
         FROM tm_nam
        WHERE     c_number = a_c_number
              AND ca_number = a_ca_number
              AND SYSTIMESTAMP <= d_timestamp)
SELECT nam, rson, url
  FROM temp
 WHERE rn = 1;

或

WITH
   temp
   AS
      (  SELECT nam, rson, url
           FROM tm_nam
          WHERE     c_number = a_c_number
                AND ca_number = a_ca_number
                AND SYSTIMESTAMP <= d_timestamp
       ORDER BY d_timestamp DESC)
SELECT nam, rson, url
  FROM temp
 WHERE ROWNUM = 1;

您的第一个和第二个查询不一样，因为第二个查询相当于：

SELECT NAM, RSON, URL
FROM   TM_CAM
WHERE  C_NUMBER     =  A_C_NUMBER
AND    CA_NUMBER    =  A_CA_NUMBER
AND    SYSTIMESTAMP <= D_TIMESTAMP
ORDER BY D_TIMESTAMP DESC
FETCH FIRST ROW WITH TIES

可以使用ROWNUM重写它：

SELECT *
FROM   (
  SELECT NAM, RSON, URL
  FROM   TM_CAM
  WHERE  C_NUMBER     =  A_C_NUMBER
  AND    CA_NUMBER    =  A_CA_NUMBER
  AND    SYSTIMESTAMP <= D_TIMESTAMP
  ORDER BY D_TIMESTAMP DESC
)
WHERE  ROWNUM = 1;

但是，如果比较执行计划，您可能会发现Oracle对FETCH FIRST ROW ONLY、ROWNUM = 1过滤或ROW_NUMBER()分析函数过滤使用非常相似（如果不完全相同）的计划，因此您可能会选择最容易维护和使用的查询。
fiddle