C# EF生成的查询执行时间太长

我有一个由Entity Framework生成的非常简单的查询，有时当我尝试运行此查询时，执行此查询几乎需要30秒以上的时间，并且我得到了超时

异常

SELECT TOP (10) 
[Extent1].[LinkID] AS [LinkID], 
[Extent1].[Title] AS [Title], 
[Extent1].[Url] AS [Url], 
[Extent1].[Description] AS [Description], 
[Extent1].[SentDate] AS [SentDate], 
[Extent1].[VisitCount] AS [VisitCount], 
[Extent1].[RssSourceId] AS [RssSourceId], 
[Extent1].[ReviewStatus] AS [ReviewStatus], 
[Extent1].[UserAccountId] AS [UserAccountId], 
[Extent1].[CreationDate] AS [CreationDate]
FROM ( SELECT [Extent1].[LinkID] AS [LinkID], [Extent1].[Title] AS [Title], [Extent1].[Url] AS [Url], [Extent1].[Description] AS [Description], [Extent1].[SentDate] AS [SentDate], [Extent1].[VisitCount] AS [VisitCount], [Extent1].[RssSourceId] AS [RssSourceId], [Extent1].[ReviewStatus] AS [ReviewStatus], [Extent1].[UserAccountId] AS [UserAccountId], [Extent1].[CreationDate] AS [CreationDate], row_number() OVER (ORDER BY [Extent1].[SentDate] DESC) AS [row_number]
    FROM [dbo].[Links] AS [Extent1]
)  AS [Extent1]
WHERE [Extent1].[row_number] > 0
ORDER BY [Extent1].[SentDate] DESC

生成查询的代码是：

public async Task<IQueryable<TEntity>> GetAsync(Expression<Func<TEntity, bool>> filter = null,
    Func<IQueryable<TEntity>, IOrderedQueryable<TEntity>> orderBy = null)
{
    return await Task.Run(() =>
    {
        IQueryable<TEntity> query = _dbSet;
        if (filter != null)
        {
            query = query.Where(filter);
        }

        if (orderBy != null)
        {
            query = orderBy(query);
        }

        return query;
    });
}

任何建议都会有帮助

更新：

以下是上述代码的用法：

var dbLinks = await _uow.LinkRespository.GetAsync(filter, orderBy);
var pagedLinks = new PagedList<Link>(dbLinks, pageNumber, PAGE_SIZE);
var vmLinks = Mapper.Map<IPagedList<LinkViewItemViewModel>>(pagedLinks);

---

你试过用这种方法链接吗

        IQueryable<TEntity> query = _dbSet;
        return query.Where(x => (filter != null ? filter : x)
                    .Where(x => (orderBy != null ? orderBy : x));

IQueryable查询=\u dbSet；
返回查询。其中（x=>（filter！=null？filter:x）
。其中（x=>（orderBy！=null？orderBy:x））；

---

我想知道这是否会改变EF创建的查询。

我猜

行号>0

会随着时间的推移而改变，因为您需要第2页、第3页等

因此，我很好奇它是否有助于创建此索引：

CREATE INDEX idx_links_SentDate_desc ON [dbo].[Links] ([SentDate] DESC)

诚然，如果它有效，它几乎是一个创可贴，你可能需要频繁地重建这个索引，因为我猜它会随着时间的推移而变得碎片化……

---

更新：检查注释！事实证明

DESC

没有任何效果，如果您的数据从低到高，应该避免使用它！

我从来没有想到您只是没有索引。经验教训-在进一步挖掘之前始终检查基础知识

---

如果不需要分页，那么查询可以简化为

SELECT TOP (10) 
    [Extent1].[LinkID] AS [LinkID], 
    [Extent1].[Title] AS [Title], 
    ...
FROM [dbo].[Links] AS [Extent1]
ORDER BY [Extent1].[SentDate] DESC

正如你所证实的，它运行得很快

显然，你确实需要分页，让我们看看我们能做些什么

当前版本之所以慢，是因为它首先扫描整个表，计算每行的行号，然后返回10行。我错了。SQL Server optimizer非常聪明。问题的根源在其他地方。请参阅下面的更新。

---

顺便说一句，正如其他人提到的，只有当

SentDate

列是唯一的时，此分页才会正确工作。如果它不是唯一的，则需要

按SentDate

排序，以及另一个唯一的列，如一些

ID

，以解决歧义

如果您不需要直接跳转到特定页面，而是始终从第1页开始，然后转到下一页、下一页等等，那么这篇优秀的文章介绍了执行此类分页的正确有效方法： 作者使用PostgreSQL进行说明，但该技术也适用于MS SQL Server。它归结为记住显示页面上最后一行的

ID

，然后在

WHERE

子句中使用该

ID

，并使用适当的支持索引检索下一页，而无需扫描之前的所有行

SQL Server 2008没有对分页的内置支持，因此我们必须使用变通方法。我将展示一种变体，它允许直接跳转到给定页面，在第一个页面上运行速度很快，但在以后的页面上运行速度会越来越慢

你的C代码中会有这些变量（

PageSize

，

PageNumber

），我把它们放在这里是为了说明这一点

DECLARE @VarPageSize int = 10; -- number of rows in each page
DECLARE @VarPageNumber int = 3; -- page numeration is zero-based

SELECT TOP (@VarPageSize)
    [Extent1].[LinkID] AS [LinkID]
    ,[Extent1].[Title] AS [Title]
    ,[Extent1].[Url] AS [Url]
    ,[Extent1].[Description] AS [Description]
    ,[Extent1].[SentDate] AS [SentDate]
    ,[Extent1].[VisitCount] AS [VisitCount]
    ,[Extent1].[RssSourceId] AS [RssSourceId]
    ,[Extent1].[ReviewStatus] AS [ReviewStatus]
    ,[Extent1].[UserAccountId] AS [UserAccountId]
    ,[Extent1].[CreationDate] AS [CreationDate]
FROM
    (
        SELECT TOP((@VarPageNumber + 1) * @VarPageSize)
            [Extent1].[LinkID] AS [LinkID]
            ,[Extent1].[Title] AS [Title]
            ,[Extent1].[Url] AS [Url]
            ,[Extent1].[Description] AS [Description]
            ,[Extent1].[SentDate] AS [SentDate]
            ,[Extent1].[VisitCount] AS [VisitCount]
            ,[Extent1].[RssSourceId] AS [RssSourceId]
            ,[Extent1].[ReviewStatus] AS [ReviewStatus]
            ,[Extent1].[UserAccountId] AS [UserAccountId]
            ,[Extent1].[CreationDate] AS [CreationDate]
        FROM [dbo].[Links] AS [Extent1]
        ORDER BY [Extent1].[SentDate] DESC
    ) AS [Extent1]
ORDER BY [Extent1].[SentDate] ASC
;

第一页是第1到10行，第二页是第11到20行，依此类推。 让我们看看当我们试图获取第四页时，即第31到40行时，这个查询是如何工作的。

PageSize=10

，

PageNumber=3

。在内部查询中，我们选择前40行。注意，我们这里不扫描整个表，我们只扫描前40行。我们甚至不需要显式的

ROW_NUMBER（）

。然后我们需要从找到的40行中选择最后10行，因此外部查询选择顶部（10）与

按

相反的方向排序。按原样，这将以相反的顺序返回第40行到第31行。您可以在客户端上将它们重新排序为正确的顺序，或者添加一个外部查询，只需按

SentDate DESC

再次对它们进行排序。如下所示：

SELECT
    [Extent1].[LinkID] AS [LinkID]
    ,[Extent1].[Title] AS [Title]
    ,[Extent1].[Url] AS [Url]
    ,[Extent1].[Description] AS [Description]
    ,[Extent1].[SentDate] AS [SentDate]
    ,[Extent1].[VisitCount] AS [VisitCount]
    ,[Extent1].[RssSourceId] AS [RssSourceId]
    ,[Extent1].[ReviewStatus] AS [ReviewStatus]
    ,[Extent1].[UserAccountId] AS [UserAccountId]
    ,[Extent1].[CreationDate] AS [CreationDate]
FROM
    (
        SELECT TOP (@VarPageSize)
            [Extent1].[LinkID] AS [LinkID]
            ,[Extent1].[Title] AS [Title]
            ,[Extent1].[Url] AS [Url]
            ,[Extent1].[Description] AS [Description]
            ,[Extent1].[SentDate] AS [SentDate]
            ,[Extent1].[VisitCount] AS [VisitCount]
            ,[Extent1].[RssSourceId] AS [RssSourceId]
            ,[Extent1].[ReviewStatus] AS [ReviewStatus]
            ,[Extent1].[UserAccountId] AS [UserAccountId]
            ,[Extent1].[CreationDate] AS [CreationDate]
        FROM
            (
                SELECT TOP((@VarPageNumber + 1) * @VarPageSize)
                    [Extent1].[LinkID] AS [LinkID]
                    ,[Extent1].[Title] AS [Title]
                    ,[Extent1].[Url] AS [Url]
                    ,[Extent1].[Description] AS [Description]
                    ,[Extent1].[SentDate] AS [SentDate]
                    ,[Extent1].[VisitCount] AS [VisitCount]
                    ,[Extent1].[RssSourceId] AS [RssSourceId]
                    ,[Extent1].[ReviewStatus] AS [ReviewStatus]
                    ,[Extent1].[UserAccountId] AS [UserAccountId]
                    ,[Extent1].[CreationDate] AS [CreationDate]
                FROM [dbo].[Links] AS [Extent1]
                ORDER BY [Extent1].[SentDate] DESC
            ) AS [Extent1]
        ORDER BY [Extent1].[SentDate] ASC
    ) AS [Extent1]
ORDER BY [Extent1].[SentDate] DESC

此查询（作为原始查询）只有当

SentDate

是唯一的时，它才能始终正常工作。如果它不是唯一的，请在

ORDER BY

中添加唯一列。例如，如果

LinkID

是唯一的，则在最内部的查询中使用

ORDER BY SentDate DESC，LinkID DESC

。在外部查询中，颠倒顺序：

ORDER BY SentDate ASC，LinkID ASC

显然，如果您想跳转到第1000页，那么内部查询将必须读取10000行，因此越是深入，速度就越慢

在任何情况下，您都需要在

SentDate

（或

SentDate，LinkID

）上有一个索引才能使其工作。如果没有索引，查询将再次扫描整个表

我不会在这里告诉你如何将这个查询转换为EF，因为我不知道。我从来没有使用过EF。可能有一种方法。而且，显然，你可以强制它使用实际的SQL，而不是尝试使用C代码

更新 执行计划比较

在我的数据库中，我有一个包含29477859行的表

EventLogErrors

，我在SQL Server 2008上比较了EF生成的

ROW_NUMBER

查询和我在这里建议的

TOP

查询。我尝试检索第四页10行。在这两种情况下，优化器都足够聪明，可以只读取40行，如您所见从执行计划中。我使用一个主键列进行排序和分页。当我使用另一个索引列进行分页时，结果是相同的，即两个变量仅读取40行。不用说，两个变量都在几分之一秒内返回结果

SELECT TOP (10) 
[Extent1].[LinkID] AS [LinkID], 
[Extent1].[Title] AS [Title], 
.
.
.
FROM [dbo].[Links] AS [Extent1]
ORDER BY [Extent1].[SentDate] DESC

带有

顶部的变体

带有

行号的变体

这一切都意味着问题的根源在其他地方。您提到您的查询运行缓慢，只是有时运行缓慢，而我最初并没有真正注意到它。对于这种症状，我会做以下操作：

• SELECT [Extent1].[LinkID] AS [LinkID] ,[Extent1].[Title] AS [Title] ,[Extent1].[Url] AS [Url] ,[Extent1].[Description] AS [Description] ,[Extent1].[SentDate] AS [SentDate] ,[Extent1].[VisitCount] AS [VisitCount] ,[Extent1].[RssSourceId] AS [RssSourceId] ,[Extent1].[ReviewStatus] AS [ReviewStatus] ,[Extent1].[UserAccountId] AS [UserAccountId] ,[Extent1].[CreationDate] AS [CreationDate] FROM ( SELECT TOP (@VarPageSize) [Extent1].[LinkID] AS [LinkID] ,[Extent1].[Title] AS [Title] ,[Extent1].[Url] AS [Url] ,[Extent1].[Description] AS [Description] ,[Extent1].[SentDate] AS [SentDate] ,[Extent1].[VisitCount] AS [VisitCount] ,[Extent1].[RssSourceId] AS [RssSourceId] ,[Extent1].[ReviewStatus] AS [ReviewStatus] ,[Extent1].[UserAccountId] AS [UserAccountId] ,[Extent1].[CreationDate] AS [CreationDate] FROM ( SELECT TOP((@VarPageNumber + 1) * @VarPageSize) [Extent1].[LinkID] AS [LinkID] ,[Extent1].[Title] AS [Title] ,[Extent1].[Url] AS [Url] ,[Extent1].[Description] AS [Description] ,[Extent1].[SentDate] AS [SentDate] ,[Extent1].[VisitCount] AS [VisitCount] ,[Extent1].[RssSourceId] AS [RssSourceId] ,[Extent1].[ReviewStatus] AS [ReviewStatus] ,[Extent1].[UserAccountId] AS [UserAccountId] ,[Extent1].[CreationDate] AS [CreationDate] FROM [dbo].[Links] AS [Extent1] ORDER BY [Extent1].[SentDate] DESC ) AS [Extent1] ORDER BY [Extent1].[SentDate] ASC ) AS [Extent1] ORDER BY [Extent1].[SentDate] DESC

  int takeNo = 20;
  int skipNo = 100;
  
  var results = db.ExecuteStoreQuery<Link>(
      "SELECT LinkID, Title, Url, Description, SentDate, VisitCount, RssSourceId, ReviewStatus, UserAccountId, CreationDate FROM Links", 
      null);
  
  results = results.OrderBy(x=> x.SentDate).Skip(skipNo).Take(takeNo);
  

  if (orderBy != null)
  {
      query = orderBy(query);
  }
  

  SELECT TOP (10) 
  [Extent1].[LinkID] AS [LinkID], 
  [Extent1].[Title] AS [Title], 
  [Extent1].[Url] AS [Url], 
  [Extent1].[Description] AS [Description], 
  [Extent1].[SentDate] AS [SentDate], 
  [Extent1].[VisitCount] AS [VisitCount], 
  [Extent1].[RssSourceId] AS [RssSourceId], 
  [Extent1].[ReviewStatus] AS [ReviewStatus], 
  [Extent1].[UserAccountId] AS [UserAccountId], 
  [Extent1].[CreationDate] AS [CreationDate]
  FROM [dbo].[Links] AS [Extent1]
  ORDER BY [Extent1].[SentDate] DESC
  

  select top 10 A.* from (
  SELECT * from
  [Extent1].[LinkID] AS [LinkID], 
  [Extent1].[Title] AS [Title], 
  [Extent1].[Url] AS [Url], 
  [Extent1].[Description] AS [Description], 
  [Extent1].[SentDate] AS [SentDate], 
  [Extent1].[VisitCount] AS [VisitCount], 
  [Extent1].[RssSourceId] AS [RssSourceId], 
  [Extent1].[ReviewStatus] AS [ReviewStatus], 
  [Extent1].[UserAccountId] AS [UserAccountId], 
  [Extent1].[CreationDate] AS [CreationDate]
  FROM [dbo].[Links] AS [Extent1] ) A
  ORDER BY A.[SentDate] DESC