sql-server-2008 - 为什么Sql Server建议在索引已经存在的情况下创建索引？

Question

我对我们的一个表运行了一个非常基本的查询，我注意到执行计划查询处理器建议我们在列上创建索引

查询是

SELECT SUM(DATALENGTH(Data))
FROM Item
WHERE Namespace = 'http://some_url/some_namespace/'

运行后得到如下信息

// The Query Processor estimates that implementing the following index could improve the query cost by 96.7211%.
CREATE NONCLUSTERED INDEX [<Name of Missing Index, sysname,>]
ON [dbo].[Item] ([Namespace])

我的问题是我已经在那个列上有了这样的索引:

CREATE NONCLUSTERED INDEX [IX_ItemNamespace] ON [dbo].[Item] 
(
    [Namespace] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]

为什么 Sql Server 推荐我在已经存在的情况下创建这样的索引？

Answer 1

索引列和排序顺序有所不同...

上一个 SO 1和 SO 2

但是，对于这个简单的事情，请尝试添加一个 INCLUDE clause使其覆盖。

CREATE NONCLUSTERED INDEX [IX_ItemNamespace] ON [dbo].[Item] ([Namespace]) INCLUDE ([Data])