（1）select * from bdorder
（2） select * from bdorder
order by orderdate desc
前者执行为15% 而后者要85% 差了5倍多！
故在用DESC 要慎用，效率太低！

典型应用：
左连接取出连接表符合条件的第一条记录
left? join bdship d(NOLOCK)
on d.orderno = c.orderno
AND d.id=(select top 1 id from bdship m where m.orderno = d.orderno order by id desc )
应改为：
left? join bdship d(NOLOCK)
on d.orderno = c.orderno
AND d.shipdate=(select max(shipdate) from bdship m where m.orderno = d.orderno? )