是否可以为运行时间复杂度为O（n）的“获取最大2”编写T-SQL查询？

Question

Obviously it is possible to find the largest 2 elements in a collection in O(n) time complexity. I wondering if this can be done in SQL Server or if any query that has an ORDER BY will always run in at least O(n)*log(n) time.

Answer 1

"if any query . . . has an ORDER BY will always run in O(n logn) time."

False. That is not "obvious" at all. The following query:

select top (2) t.*
from t
order by t.size desc;

Will run in O(1) time (I think) with a clustered index on t(size) and in O(log n) time with a b-tree index. For normal data sizes, O(log n) is pretty close to constant time -- just put in a constant of 20 or so.