1

我已经阅读了很多关于索引和它们之间的差异的内容。现在我正在我的项目中进行查询优化。我创建了非聚集索引，应该在查询执行时使用，但事实并非如此。详情如下：

桌子：

在此处输入图像描述

指数：

CREATE NONCLUSTERED INDEX [_IXProcedure_Deleted_Date] ON [por].[DailyAsset]
(
    [Deleted] ASC,
    [Date] DESC
)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

实体框架生成的查询：

exec sp_executesql N'SELECT 
[Project1].[C1] AS [C1], 
[Project1].[AssetId] AS [AssetId], 
[Project1].[Active] AS [Active], 
[Project1].[Date] AS [Date]
FROM ( SELECT 
    [Extent1].[AssetId] AS [AssetId], 
    [Extent1].[Active] AS [Active], 
    [Extent1].[Date] AS [Date], 
    1 AS [C1]
    FROM [por].[DailyAsset] AS [Extent1]
    WHERE (0 = [Extent1].[Deleted]) AND ([Extent1].[Date] < @p__linq__0)
)  AS [Project1]
ORDER BY [Project1].[Date] DESC',N'@p__linq__0 datetime2(7)',@p__linq__0='2014-05-01 00:00:00'

执行计划：

在此处输入图像描述

缺少索引详细信息：

The Query Processor estimates that implementing the following index could improve the query cost by 23.8027%.


CREATE NONCLUSTERED INDEX [<Name of Missing Index, sysname,>]
ON [por].[DailyAsset] ([Deleted],[Date])
INCLUDE ([AssetId],[Active])

我知道通过将 AssetId 和 Active 列包含到索引中，将使用索引。

现在，为什么没有列包含它就不能工作？

这是另一个查询的简化示例，其中所有列都作为结果被提取。（强制）索引查找使用的唯一解决方案是将所有列包含在索引中，它具有相同的估计子树成本（显然）。

另一个烦人的问题是排序无知。日期列在索引中并设置为 DESCENDING。它被完全忽略了，当然，排序操作在执行计划中占据了昂贵的位置。

更新 1：

正如@Jayachandran 指出的那样，IndexSeek + KeyLookUp 应该在上面的查询中使用，但是覆盖索引有据可查，并且它假定应该包括 AssetId 和 Active 列。我同意这一点。

我正在创建 UPDATE 1 以在下面的查询中演示覆盖索引的有用性。同一张表，更大的结果集。据我所知，不应在索引中使用单个列，并且为 Date 和 Deleted 列创建索引。

exec sp_executesql N'SELECT 
[Project1].[DailyAssetId] AS [DailyAssetId], 
[Project1].[AssetId] AS [AssetId], 
[Project1].[CreatedByUserId] AS [CreatedByUserId], 
[Project1].[UpdatedByUserId] AS [UpdatedByUserId], 
[Project1].[TimeCreated] AS [TimeCreated], 
[Project1].[TimeUpdated] AS [TimeUpdated], 
[Project1].[Deleted] AS [Deleted], 
[Project1].[TimeDeleted] AS [TimeDeleted], 
[Project1].[DeletedByUserId] AS [DeletedByUserId], 
[Project1].[Active] AS [Active], 
[Project1].[Date] AS [Date], 
[Project1].[Quantity] AS [Quantity], 
[Project1].[TotalBookValue] AS [TotalBookValue], 
[Project1].[CostPrice] AS [CostPrice], 
[Project1].[CostValue] AS [CostValue], 
[Project1].[FairPrice] AS [FairPrice], 
[Project1].[FairValue] AS [FairValue], 
[Project1].[UnsettledQuantity] AS [UnsettledQuantity], 
[Project1].[UnsettledValue] AS [UnsettledValue], 
[Project1].[SettlementDate] AS [SettlementDate], 
[Project1].[EffectiveDate] AS [EffectiveDate], 
[Project1].[PortfolioId] AS [PortfolioId]
FROM ( SELECT 
    [Extent1].[DailyAssetId] AS [DailyAssetId], 
    [Extent1].[AssetId] AS [AssetId], 
    [Extent1].[CreatedByUserId] AS [CreatedByUserId], 
    [Extent1].[UpdatedByUserId] AS [UpdatedByUserId], 
    [Extent1].[TimeCreated] AS [TimeCreated], 
    [Extent1].[TimeUpdated] AS [TimeUpdated], 
    [Extent1].[Deleted] AS [Deleted], 
    [Extent1].[TimeDeleted] AS [TimeDeleted], 
    [Extent1].[DeletedByUserId] AS [DeletedByUserId], 
    [Extent1].[Active] AS [Active], 
    [Extent1].[Date] AS [Date], 
    [Extent1].[Quantity] AS [Quantity], 
    [Extent1].[TotalBookValue] AS [TotalBookValue], 
    [Extent1].[CostPrice] AS [CostPrice], 
    [Extent1].[CostValue] AS [CostValue], 
    [Extent1].[FairPrice] AS [FairPrice], 
    [Extent1].[FairValue] AS [FairValue], 
    [Extent1].[UnsettledQuantity] AS [UnsettledQuantity], 
    [Extent1].[UnsettledValue] AS [UnsettledValue], 
    [Extent1].[SettlementDate] AS [SettlementDate], 
    [Extent1].[EffectiveDate] AS [EffectiveDate], 
    [Extent1].[PortfolioId] AS [PortfolioId]
    FROM [por].[DailyAsset] AS [Extent1]
    WHERE (0 = [Extent1].[Deleted]) AND ([Extent1].[Date] < @p__linq__0)
)  AS [Project1]
ORDER BY [Project1].[Date] DESC',N'@p__linq__0 datetime2(7)',@p__linq__0='2014-05-01 00:00:00'
4

2 回答 2

5

在这种情况下，扫描和查找（使用键查找）的区别在于返回的行数。体积太大，因此优化器选择了一个更便宜的计划 - 只需扫描整个表。这将比使用 NC 索引更快。

想象一下，如果您强制它使用 NC 索引，并且它必须对表中 40% 的行进行键查找。这就像一个 foreach 循环执行了很多次。所以 SQL 选择只扫描表，因为它比循环更快。

关于您关于如何考虑其他查询中可能包含的其他列的问题，实际上有几个选择。您可以创建一个包含最常用列的覆盖索引，也可以更改主键以使其朝向最常用的访问路径。即按日期，删除和唯一性的标识列。

另一方面，对主键使用 guid 会导致聚集索引和所有其他索引出现各种问题（因为 PK 的键将包含在所有其他索引中）。guid 的随机排序导致行以随机顺序插入到页面中。因为索引是有序的，所以必须不断地拆分页面以考虑新行。创建一个自然递增的索引会好得多，这也可能有助于解决上述问题，具体取决于编写的查询类型。

于 2014-09-08T13:21:00.360 回答
1

特定查询的理想索引是（1）WHERE子句中的所有字段都在索引中，（2）SELECT子句中的所有字段都包含在索引中。如果 (1) 不满足，SQL Server 将权衡访问多个索引的成本，并选择它认为最快的一个；如果 (2) 不满足，则意味着昂贵的 Key Lookup 操作。如果索引具有非常高的选择性（重复值很少）， SQL Server可能会认为它是值得的。

在您的情况下，显然不满足条件（2）。SQL Server 认为 Key Lookup 操作与聚集索引扫描相比成本太高，因此选择了后者。您可以强制 SQL Server 使用特定索引，但我不知道如何使用 Entity Framework 执行此操作。

如果此查询对您来说必须很快，请按照 SQL Server 的说明创建索引。

于 2014-09-08T13:27:44.870 回答