SQL Server选择了非聚集索引而不是聚集索引

聚集索引不仅包含索引的key值，还包含表数据；

非聚集索引只包含索引的key值。

SQL Server在有些情况下，有聚集索引和非聚集索引存在时，会选择走非聚集索引，而不走聚集索引。

例子如下：

在 SQL Server 的adventureWorks数据库下，运行如下语句：

select DepartmentID,Name from HumanResources.Department

Department表上有两个索引，一个是departmentid上的聚集索引，另一个是name上的非聚集索引；

该语句的运行结果如下：

DepartmentID Name

12 Document Control

1 Engineering

16 Executive

14 Facilities and Maintenance

10 Finance

9 Human Resources

11 Information Services

4 Marketing

7 Production

8 Production Control

5 Purchasing

13 Quality Assurance

6 Research and Development

3 Sales

15 Shipping and Receiving

2 Tool Design

可以看到结果是按在name的升序排列的，说明使用的是name上的非聚集索引。

如果存在另外一个没有索引的列，则会选择走departmentid上的聚集索引，因为sql server会认为这样的速度比较快。

select DepartmentID,Name,GroupName from HumanResources.Department

DepartmentID Name GroupName

1 Engineering Research and Development

2 Tool Design Research and Development

3 Sales Sales and Marketing

4 Marketing Sales and Marketing

5 Purchasing Inventory Management

6 Research and Development Research and Development

7 Production Manufacturing

8 Production Control Manufacturing

9 Human Resources Executive General and Administration

10 Finance Executive General and Administration

11 Information Services Executive General and Administration

12 Document Control Quality Assurance

13 Quality Assurance Quality Assurance

14 Facilities and Maintenance Executive General and Administration

15 Shipping and Receiving Inventory Management

16 Executive Executive General and Administration

但是，如果把groupname也加入name列上的非聚集索引后，会发现，SQL Server又走非聚集索引了。

DepartmentID Name GroupName

12 Document Control Quality Assurance

1 Engineering Research and Development

16 Executive Executive General and Administration

14 Facilities and Maintenance Executive General and Administration

10 Finance Executive General and Administration

9 Human Resources Executive General and Administration

11 Information Services Executive General and Administration

4 Marketing Sales and Marketing

7 Production Manufacturing

8 Production Control Manufacturing

5 Purchasing Inventory Management

13 Quality Assurance Quality Assurance

6 Research and Development Research and Development

3 Sales Sales and Marketing

15 Shipping and Receiving Inventory Management

2 Tool Design Research and Development

造成这种现象的原因是由于非聚集索引只存储key值，所以会使用更少的page，这样sql
server在生成执行计划时，便会优先考虑这个非聚集索引，因为这样的能更减少I/O。

但实际上，从执行计划分析，这个例子中走聚集索引与非聚集索引的CPU，I/O代价是完全一致的，估计是SQL Server设定的某种优先级吧。