为什么这个查询键查找？

Question

我想了解为什么这个查询是键查找，即使我添加了我在索引中选择的所有列？

这是查询

SELECT TOP 1
    ART.Id, 
    ART.Title, 
    ART.InitialTitle, 
    ART.Summary,
    ART.IsCategoryFirst,
    ART.RootCategoryId, 
    CAT.Name, 
    ART.OldFacebookCommentsUrl, 
    ICO.CssClass,
    ART.TopicName, 
    ART.MainArticlePhoto, 
    ART.FrontPagePhoto, 
    ART.PublishDate
FROM Articles ART WITH (NOLOCK)
INNER JOIN Categories CAT WITH (NOLOCK) ON CAT.Id = ART.RootCategoryId 
LEFT JOIN ArticleIcons ICO WITH (NOLOCK) ON ICO.Id = ART.IconId
WHERE ART.RootCategoryId = 3
  AND ART.PublishDate < GETDATE() 
  AND ART.Active = 1
  AND IsCategoryFirst = 1

这是应该涵盖它的索引

CREATE NONCLUSTERED INDEX [SmallArticleResponse] ON [dbo].[Articles]
(
    [RootCategoryId] ASC,
    [Active] ASC,
    [PublishDate] DESC,
    [Coefficient] DESC
)
INCLUDE (   [Id],
    [Title],
    [InitialTitle],
    [OldFacebookCommentsUrl],
    [Summary],
    [IconId],
    [TopicName],
    [FrontPagePhoto],
    [MainArticlePhoto],
    [FacebookPhoto],
    [IsCategoryFirst]) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

这是它的执行计划和信息。

Answer 1

因为 IsCategoryFirst 列很可能不在键中。 And/or 因为 IsCategoryFirst 上有一个单独的（单列？）索引，它正在使用它。

为什么这个查询键查找？

Why does this query a key lookup?

sql-server

sql-optimization

sql-execution-plan