如何执行非聚集索引查找而不是聚集索引扫描 | 如何执行非聚集索引查找而不是聚集索引扫描

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问题描述

我测试了非聚集索引的好处.

I test the benefits of nonclustered indexes.

我使用db AdventureWorks当我执行查询时:

I use db AdventureWorksWhen i execute the query:

SELECT [address].City, [address].[AddressLine1]
FROM [AdventureWorks].[Person].[Address] as [address]
WHERE [address].City = 'Seattle'

我在执行计划标签中看到

I see in execution plan tab

/*
Missing Index Details from SQLQuery3.sql -
The Query Processor estimates that implementing the following index could improve the query cost by 97.9636%.
*/

/*
USE [AdventureWorks]
GO
CREATE NONCLUSTERED INDEX [<Name of Missing Index, sysname,>]
ON [Person].[Address] ([City])

GO
*/

我在执行的普通选项卡图标中看到集群索引扫描"，并且我知道这很糟糕，因为索引查找更好

And i see in the execution plain tab icon "Clustered index scan" and i know that it is bad because index seek is better

但是当我执行查询

USE [AdventureWorks]
GO
CREATE NONCLUSTERED INDEX CityIdx
ON [Person].[Address] ([City])

GO

我仍然看到执行中的普通标签集群索引扫描".为什么不聚集索引搜索"?是否应该是聚集索引搜索"?在哪种情况下应为聚集索引查找".

推荐答案

您正在点击索引临界点: City ='Seattle'的条目太多，以至于每个条目都找不到 AddressLine1 在聚集索引中.对于这个特定查询，一种方法是包括计划的列:

You are hitting the index tipping point: there are simply too many entries with City = 'Seattle' to bother seeking, for each one, the AddressLine1 in the clustered index. One approach, for this particular query, is to include the projected column:

CREATE NONCLUSTERED INDEX CityIdx
ON [Person].[Address] ([City])
INCLUDE ([AddressLine1]);

但是，这隐藏了 real 问题，即为什么您是否有兴趣在这样的非选择性谓词上选择 all 行?该应用程序不应发出此类请求.

But that hides the real issue, namely why are are you interested in selecting all rows on such a non-selective predicate? The application should not make such requests.

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