你能看一下吗http://sqlfiddle.com/#!18/7ad28/8，帮助我理解为什么添加where条件会在seek from scan上带来索引？根据我的（错误的）理解，它不应该有任何区别，因为它是一个更大的条件，应该导致扫描。
我还在下面粘贴有问题的表脚本和查询

CREATE TABLE [dbo].[Mappings]
(
[MappingID] [smallint] NOT NULL IDENTITY(1, 1),
[ProductID] [smallint] NOT NULL,
[CategoryID] [smallint] NOT NULL
) 
GO
ALTER TABLE [dbo].[Mappings] ADD CONSTRAINT [pk_Mappings_MappingID] PRIMARY KEY CLUSTERED ([MappingID]) WITH (DATA_COMPRESSION = PAGE) ON [PRIMARY]
GO
CREATE NONCLUSTERED INDEX [nc_Mappings_ProductIDCategoryID] ON [dbo].[Mappings] ([ProductID], [CategoryID]) WITH (DATA_COMPRESSION = PAGE) ON [PRIMARY]
GO

CREATE TABLE [dbo].[CustomerProducts]
(
[CustomerID] [bigint] NOT NULL,
[ProductID] [smallint] NOT NULL,
[SomeDate] [datetimeoffset] (0) NULL,
[SomeAttribute] [bigint] NULL
) 
GO
ALTER TABLE [dbo].[CustomerProducts] ADD CONSTRAINT [pk_CustomerProducts_ProductIDCustomerID] PRIMARY KEY CLUSTERED ([ProductID], [CustomerID]) ON [PRIMARY]
GO

--SCAN [tempdb].[dbo].[Mappings].[nc_Mappings_ProductIDCategoryID].(NonClustered)   
SELECT b.[SomeDate],
       b.[SomeAttribute]
FROM dbo.[Mappings] a
    INNER JOIN dbo.CustomerProducts b
        ON a.[ProductID] = b.[ProductID]
Where b.CustomerID = 88;

--SEEK [tempdb].[dbo].[Mappings].[nc_Mappings_ProductIDCategoryID].(NonClustered)   
SELECT b.[SomeDate],
       b.[SomeAttribute]
FROM dbo.[Mappings] a
    INNER JOIN dbo.CustomerProducts b
        ON a.[ProductID] = b.[ProductID]
           AND b.CustomerID = 88
Where a.[ProductID] > 0;