mysql> select count(*) from (select * from t1 order by id )t;
+----------+
| count(*) |
+----------+
|   499999 |
+----------+
1 row in set (11.86 sec)

mysql> select count(*) from (select * from t1 order by id desc)t;
+----------+
| count(*) |
+----------+
|   499999 |
+----------+
1 row in set (11.73 sec)

mysql>

------解决方案--------------------
探讨

新手刚用mysql，请问一下一张a表3W条的记录

在a表的xxx字段建立索引
create index a_xxx  on a(xxx desc);
然后

select * from table order by XXX 只需要 0.02秒
为什么用
select * from table order by XXX desc就需要 6-7秒

不解..求指点

是不是没……