Mysql rand()函数

查询语句
SELECT * FROM tablename ORDER BY RAND() LIMIT 100
查询效率极其低下，解释如下：
works for small tables, but once the tables grow larger than 300,000
records or so this will be very slow because MySQL will have to process
ALL the entries from the table, order them randomly and then return the
first row of the ordered result, and this sorting takes long time.
Instead you can do it like this (atleast if you have an auto_increment PK):
可见得扫描全表再做随机排序，效率怎么能不低。
网上的解决办法
SELECT * FROM 'table'
WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM 'table')))　
ORDER BY id LIMIT 100;
通过 max(id) * rand()来确定随机数，不过个人认为这个办法有很大的局限
1）
RAND()*MAX(id)的值如果是最大id值，那怎么可能查的出来100条数据
2）
再者，如果id不是连续的，比如 1，2，3，1000，那这个查询也有问题
难道RAND()没有很好的解决方案?