.net 为什么LINQ JOIN比链接WHERE快得多？

我最近升级到VS2010，正在使用LINQtoDataSet。我有一个用于授权的强类型数据集，它位于ASP.NET Web应用程序的HttpCache中

所以我想知道检查用户是否被授权做某事的最快方法是什么。是我的数据模型和一些其他信息，如果有人感兴趣

我检查了3种方法：

直接数据库

使用Where条件作为“Join”语法的LINQ查询

使用连接的LINQ查询-语法

以下是对每个函数调用1000次的结果：

1.迭代：

Public Function hasAccessDS_Join(ByVal accessRule As String) As Boolean
    Dim userID As Guid = DirectCast(Membership.GetUser.ProviderUserKey, Guid)
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule _
                Join roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule _
                On accRule.idAccessRule Equals roleAccRule.fiAccessRule _
                Join role In Authorization.dsAuth.aspnet_Roles _
                On role.RoleId Equals roleAccRule.fiRole _
                Join userRole In Authorization.dsAuth.aspnet_UsersInRoles _
                On userRole.RoleId Equals role.RoleId _
                Where userRole.UserId = userID And accRule.RuleName.Contains(accessRule)
                Select accRule.idAccessRule
    Return query.Any
End Function

42841519秒

1157796925秒

2024749秒

2.迭代：

Public Function hasAccessDS_Join(ByVal accessRule As String) As Boolean
    Dim userID As Guid = DirectCast(Membership.GetUser.ProviderUserKey, Guid)
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule _
                Join roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule _
                On accRule.idAccessRule Equals roleAccRule.fiAccessRule _
                Join role In Authorization.dsAuth.aspnet_Roles _
                On role.RoleId Equals roleAccRule.fiRole _
                Join userRole In Authorization.dsAuth.aspnet_UsersInRoles _
                On userRole.RoleId Equals role.RoleId _
                Where userRole.UserId = userID And accRule.RuleName.Contains(accessRule)
                Select accRule.idAccessRule
    Return query.Any
End Function

31954857秒

8497047秒

15783397秒

3.迭代：

Public Function hasAccessDS_Join(ByVal accessRule As String) As Boolean
    Dim userID As Guid = DirectCast(Membership.GetUser.ProviderUserKey, Guid)
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule _
                Join roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule _
                On accRule.idAccessRule Equals roleAccRule.fiAccessRule _
                Join role In Authorization.dsAuth.aspnet_Roles _
                On role.RoleId Equals roleAccRule.fiRole _
                Join userRole In Authorization.dsAuth.aspnet_UsersInRoles _
                On userRole.RoleId Equals role.RoleId _
                Where userRole.UserId = userID And accRule.RuleName.Contains(accessRule)
                Select accRule.idAccessRule
    Return query.Any
End Function

27922143秒

978713267秒

18432163秒

平均值：

Public Function hasAccessDS_Join(ByVal accessRule As String) As Boolean
    Dim userID As Guid = DirectCast(Membership.GetUser.ProviderUserKey, Guid)
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule _
                Join roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule _
                On accRule.idAccessRule Equals roleAccRule.fiAccessRule _
                Join role In Authorization.dsAuth.aspnet_Roles _
                On role.RoleId Equals roleAccRule.fiRole _
                Join userRole In Authorization.dsAuth.aspnet_UsersInRoles _
                On userRole.RoleId Equals role.RoleId _
                Where userRole.UserId = userID And accRule.RuleName.Contains(accessRule)
                Select accRule.idAccessRule
    Return query.Any
End Function

数据库：34239506333秒

其中：995404964秒

加入：1815435秒

为什么Join版本比where语法快得多，这使得它毫无用处，尽管作为LINQ新手，它看起来是最清晰的。还是我在询问中遗漏了什么

以下是LINQ查询，我跳过数据库：

其中：

Public Function hasAccessDS_Where(ByVal accessRule As String) As Boolean
    Dim userID As Guid = DirectCast(Membership.GetUser.ProviderUserKey, Guid)
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule, _
                roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule, _
                role In Authorization.dsAuth.aspnet_Roles, _
                userRole In Authorization.dsAuth.aspnet_UsersInRoles _
                Where accRule.idAccessRule = roleAccRule.fiAccessRule _
                And roleAccRule.fiRole = role.RoleId _
                And userRole.RoleId = role.RoleId _
                And userRole.UserId = userID And accRule.RuleName.Contains(accessRule)
                Select accRule.idAccessRule
    Return query.Any
End Function

加入：

Public Function hasAccessDS_Join(ByVal accessRule As String) As Boolean
    Dim userID As Guid = DirectCast(Membership.GetUser.ProviderUserKey, Guid)
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule _
                Join roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule _
                On accRule.idAccessRule Equals roleAccRule.fiAccessRule _
                Join role In Authorization.dsAuth.aspnet_Roles _
                On role.RoleId Equals roleAccRule.fiRole _
                Join userRole In Authorization.dsAuth.aspnet_UsersInRoles _
                On userRole.RoleId Equals role.RoleId _
                Where userRole.UserId = userID And accRule.RuleName.Contains(accessRule)
                Select accRule.idAccessRule
    Return query.Any
End Function

先谢谢你

---

编辑：在对两个查询进行一些改进以获得更有意义的性能值后，联接的优势甚至比以前大了许多倍：

加入：

Public Overloads Shared Function hasAccessDS_Join(ByVal userID As Guid, ByVal idAccessRule As Int32) As Boolean
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule _
                   Join roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule _
                   On accRule.idAccessRule Equals roleAccRule.fiAccessRule _
                   Join role In Authorization.dsAuth.aspnet_Roles _
                   On role.RoleId Equals roleAccRule.fiRole _
                   Join userRole In Authorization.dsAuth.aspnet_UsersInRoles _
                   On userRole.RoleId Equals role.RoleId _
                   Where accRule.idAccessRule = idAccessRule And userRole.UserId = userID
             Select role.RoleId
    Return query.Any
End Function

Public Overloads Shared Function hasAccessDS_Where(ByVal userID As Guid, ByVal idAccessRule As Int32) As Boolean
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule, _
           roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule, _
           role In Authorization.dsAuth.aspnet_Roles, _
           userRole In Authorization.dsAuth.aspnet_UsersInRoles _
           Where accRule.idAccessRule = roleAccRule.fiAccessRule _
           And roleAccRule.fiRole = role.RoleId _
           And userRole.RoleId = role.RoleId _
           And accRule.idAccessRule = idAccessRule And userRole.UserId = userID
           Select role.RoleId
    Return query.Any
End Function

其中：

Public Overloads Shared Function hasAccessDS_Join(ByVal userID As Guid, ByVal idAccessRule As Int32) As Boolean
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule _
                   Join roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule _
                   On accRule.idAccessRule Equals roleAccRule.fiAccessRule _
                   Join role In Authorization.dsAuth.aspnet_Roles _
                   On role.RoleId Equals roleAccRule.fiRole _
                   Join userRole In Authorization.dsAuth.aspnet_UsersInRoles _
                   On userRole.RoleId Equals role.RoleId _
                   Where accRule.idAccessRule = idAccessRule And userRole.UserId = userID
             Select role.RoleId
    Return query.Any
End Function

Public Overloads Shared Function hasAccessDS_Where(ByVal userID As Guid, ByVal idAccessRule As Int32) As Boolean
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule, _
           roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule, _
           role In Authorization.dsAuth.aspnet_Roles, _
           userRole In Authorization.dsAuth.aspnet_UsersInRoles _
           Where accRule.idAccessRule = roleAccRule.fiAccessRule _
           And roleAccRule.fiRole = role.RoleId _
           And userRole.RoleId = role.RoleId _
           And accRule.idAccessRule = idAccessRule And userRole.UserId = userID
           Select role.RoleId
    Return query.Any
End Function

1000次呼叫的结果（在速度更快的计算机上）

加入| 2。在哪里

1.迭代：

Public Function hasAccessDS_Join(ByVal accessRule As String) As Boolean
    Dim userID As Guid = DirectCast(Membership.GetUser.ProviderUserKey, Guid)
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule _
                Join roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule _
                On accRule.idAccessRule Equals roleAccRule.fiAccessRule _
                Join role In Authorization.dsAuth.aspnet_Roles _
                On role.RoleId Equals roleAccRule.fiRole _
                Join userRole In Authorization.dsAuth.aspnet_UsersInRoles _
                On userRole.RoleId Equals role.RoleId _
                Where userRole.UserId = userID And accRule.RuleName.Contains(accessRule)
                Select accRule.idAccessRule
    Return query.Any
End Function

00713669秒

127395299秒

2.迭代：

Public Function hasAccessDS_Join(ByVal accessRule As String) As Boolean
    Dim userID As Guid = DirectCast(Membership.GetUser.ProviderUserKey, Guid)
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule _
                Join roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule _
                On accRule.idAccessRule Equals roleAccRule.fiAccessRule _
                Join role In Authorization.dsAuth.aspnet_Roles _
                On role.RoleId Equals roleAccRule.fiRole _
                Join userRole In Authorization.dsAuth.aspnet_UsersInRoles _
                On userRole.RoleId Equals role.RoleId _
                Where userRole.UserId = userID And accRule.RuleName.Contains(accessRule)
                Select accRule.idAccessRule
    Return query.Any
End Function

00492458秒

123885925秒

3.迭代：

Public Function hasAccessDS_Join(ByVal accessRule As String) As Boolean
    Dim userID As Guid = DirectCast(Membership.GetUser.ProviderUserKey, Guid)
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule _
                Join roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule _
                On accRule.idAccessRule Equals roleAccRule.fiAccessRule _
                Join role In Authorization.dsAuth.aspnet_Roles _
                On role.RoleId Equals roleAccRule.fiRole _
                Join userRole In Authorization.dsAuth.aspnet_UsersInRoles _
                On userRole.RoleId Equals role.RoleId _
                Where userRole.UserId = userID And accRule.RuleName.Contains(accessRule)
                Select accRule.idAccessRule
    Return query.Any
End Function

00501982秒

133474216秒

平均值：

Public Function hasAccessDS_Join(ByVal accessRule As String) As Boolean
    Dim userID As Guid = DirectCast(Membership.GetUser.ProviderUserKey, Guid)
    Dim query = From accRule In Authorization.dsAuth.aspnet_AccessRule _
                Join roleAccRule In Authorization.dsAuth.aspnet_RoleAccessRule _
                On accRule.idAccessRule Equals roleAccRule.fiAccessRule _
                Join role In Authorization.dsAuth.aspnet_Roles _
                On role.RoleId Equals roleAccRule.fiRole _
                Join userRole In Authorization.dsAuth.aspnet_UsersInRoles _
                On userRole.RoleId Equals role.RoleId _
                Where userRole.UserId = userID And accRule.RuleName.Contains(accessRule)
                Select accRule.idAccessRule
    Return query.Any
End Function

加入：00569367秒

其中：128251813秒

---

Join的速度快225倍

---

结论：避免在哪里指定关系，并尽可能使用联接（一般而言，在和

Linq to Objects

中都是如此）。

联接速度要快得多，因为该方法知道如何组合表以将结果减少到相关的组合中。使用

Where

指定关系时，必须创建每个可能的组合，然后测试条件以查看哪些组合相关

---

Join

方法可以设置一个哈希表，用作将两个表快速压缩到一起的索引，而

Where

方法在创建所有组合后运行，因此，它不能预先使用任何技巧来减少组合。

您真正需要知道的是为这两条语句创建的sql。有几种方法可以实现，但最简单的是使用LinqPad。查询结果正上方有几个按钮将更改为sql。这会给你比其他任何东西更多的信息

不过你在那里分享了很多信息

第一种方法（数据库中的SQL查询）非常有效，因为数据库知道如何执行连接。但将其与其他方法进行比较并没有意义，因为它们直接在内存中工作（Linq到数据集）

具有多个表和

条件的查询，其中

条件实际执行所有表的笛卡尔乘积，然后过滤满足该条件的行。这意味着对每个行组合（n1*n2*n3*n4）评估

，其中

条件

Join

操作符从第一个表中获取行，然后仅从第二个表中获取具有匹配键的行，然后仅从第三个表中获取具有匹配键的行，依此类推。这更有效，因为它不需要执行那么多操作

---

非常感谢。编译器/运行时是否没有像dbms中那样的隐式优化？不应该不可能看出where关系实际上是一个连接。一个好的RDBMS应该确实发现where条件是对两个唯一列的相等性的测试，并将其视为一个连接。@Tim Schelter，当您使用Linq to SQL或Linq to Entities时，有这样一种优化，因为生成的SQL查询被DBMS视为联接。但在这种情况下，您使用Linq到数据集，没有到数据集的转换SQL@Tim：LINQ to数据集实际上使用LINQ to对象。因此，只有使用

join

关键字才能捕获真正的连接，因为没有对查询进行运行时分析来生成类似于执行计划的任何内容。您还会注意到，基于LINQ的联接只能容纳单列Equijoin。@Adam，这不完全正确：您可以使用匿名类型使用多个键进行Equijoin:

。。。在new{f1.Key1，f1.Key2}上，等于new{f2.Key1，f2.Key2}

感谢LinqPad提示。实际上，我的两个查询是linQ到数据集的内存查询，因此我假设没有生成SQL。通常情况下，数据库管理系统会对其进行优化。感谢您澄清背景。db方法并不是这个问题的一部分，但我很感兴趣地看到内存方法是否真的更快。我假设.net会以某种方式优化

where

-查询，就像dbms一样。实际上，

JOIN

的速度甚至比

WHERE

（上一次编辑）快225倍。对于阅读本文并正在使用LinqToSQL的其他人，如果他们认为更改所有连接位置可能会更好，请务必阅读THomas Levesque的评论，他在评论中说“当您使用Linq to SQL或Linq to Entities时，会有这样的优化，因为生成的SQL查询