This can cause faster query performance 这可以加快查询速度。
2.
Including the nonkey columns can speed queries that can acquire all the data they need from the index without having to access the data rows and can exceed the current index size limitations of a maximum of 16 key columns and a maximum index key size of 900 bytes 包含非键列可以加快查询，使其不必访问数据行就能从索引中获取全部所需数据，并且可以突破对当前索引大小的最多16个键列和最大索引键大小900个字节的限制。
