为什么Sql Server建议在已经存在的情况下创建索引？

Question

我跑了一个很基本的查询对我们的表之一，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，我收到以下消息

// 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，我已经在该列这样的索引：

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建议我在创建这样的索引时已经存在？

Answer 1

索引列和排序顺序有所作为......

上一页SO 1和SO 2

然而，对于这个东西简单的尝试添加INCLUDE clause，使其覆盖。

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