On the other hand , database engine tuning advisor recommendations include ordering of the key columns for indexes to optimize query performance 另一方面，数据库引擎优化顾问建议包括对索引键列进行排序，以优化查询性能。
A unique index ensures that the index key contains no duplicate values and therefore every row in the table or view is in some way unique 唯一索引确保索引键不包含重复的值，因此，表或视图中的每一行在某种程度上是唯一的。
The clustered index is implemented as a b - tree index structure that supports fast retrieval of the rows , based on their clustered index key values 聚集索引按b树索引结构实现， b树索引结构支持基于聚集索引键值对行进行快速检索。
A nonclustered index contains the nonclustered index key values and each key value entry has a pointer to the data row that contains the key value 非聚集索引包含非聚集索引键值，并且每个键值项都有指向包含该键值的数据行的指针。
When the unique index key columns of a table are nullable , and the table is on the inner side of an outer join , the index is not supported by browse mode 如果表的唯一索引键列可为空，并且表在外部联接的内侧，则浏览模式不支持索引。
Index key columns , excluding nonkeys , must follow the existing index size restrictions of 16 key columns maximum , and a total index key size of 900 bytes 索引键列（不包括非键）必须遵守现有索引大小的限制（最大键列数为16 ，总索引键大小为900字节） 。
