明确加载？它与Lazy loading和Eager loading有什么不同？

Question

我是实体框架的新手，我在Entity框架中阅读了一些关于不同上下文加载机制的文章。默认情况下启用延迟加载但是如果你的web和sql服务器之间存在高ping，那么你将使用Eager加载，而不是使用延迟加载1-by-1加载相关项目。但有一件事让我很困惑是明确加载。它通过禁用延迟加载来完成，但它本身甚至不是Eager loading ..我试图为我的研究做点什么....

private static void DifferentLoading()
        {

            //Lazy - by default
            using (var ctx = new NorthwindDBContext())
            {
                var query = ctx.Categories.Take(3);
                foreach (var category in query)
                {
                    Console.WriteLine(category.CategoryName);
                    foreach (var product in category.Products)
                    {
                        Console.WriteLine(product.ProductName);
                    }

                }
            }
            //Eager loading 
            using (var ctx = new NorthwindDBContext())
            {
                ctx.Configuration.LazyLoadingEnabled = false;
                var query = ctx.Categories.Include("Products").Take(3); // .Include() to load forcefully 
                foreach (var category in query)
                {
                    Console.WriteLine(category.CategoryName);
                    foreach (var product in category.Products)
                    {
                        Console.WriteLine(product.ProductName);
                    }

                }
            }
            //Explicit loading -- On Demand load
            using (var ctx = new NorthwindDBContext())
            {
                ctx.Configuration.LazyLoadingEnabled = false;
                var query = ctx.Categories.Take(3);
                foreach (var category in query)
                {
                    Console.WriteLine(category.CategoryName);
                    ctx.Entry(category).Collection(c => c.Products).Load(); // .Include() to load explicitely  
                    foreach (var product in category.Products)
                    {
                        Console.WriteLine(product.ProductName);
                    }

                }
            }

        }

我发现针对不同场景触发了不同的sql

1）懒惰

SELECT TOP (3) 
    [c].[CategoryID] AS [CategoryID], 
    [c].[CategoryName] AS [CategoryName], 
    [c].[Description] AS [Description], 
    [c].[Picture] AS [Picture]
    FROM [dbo].[Categories] AS [c]
GO

-- Region Parameters
DECLARE @EntityKeyValue1 Int = 1
-- EndRegion
SELECT 
    [Extent1].[ProductID] AS [ProductID], 
    [Extent1].[ProductName] AS [ProductName], 
    [Extent1].[SupplierID] AS [SupplierID], 
    [Extent1].[CategoryID] AS [CategoryID], 
    [Extent1].[QuantityPerUnit] AS [QuantityPerUnit], 
    [Extent1].[UnitPrice] AS [UnitPrice], 
    [Extent1].[UnitsInStock] AS [UnitsInStock], 
    [Extent1].[UnitsOnOrder] AS [UnitsOnOrder], 
    [Extent1].[ReorderLevel] AS [ReorderLevel], 
    [Extent1].[Discontinued] AS [Discontinued]
    FROM [dbo].[Products] AS [Extent1]
    WHERE [Extent1].[CategoryID] = @EntityKeyValue1
GO

-- Region Parameters
DECLARE @EntityKeyValue1 Int = 2
-- EndRegion
SELECT 
    [Extent1].[ProductID] AS [ProductID], 
    [Extent1].[ProductName] AS [ProductName], 
    [Extent1].[SupplierID] AS [SupplierID], 
    [Extent1].[CategoryID] AS [CategoryID], 
    [Extent1].[QuantityPerUnit] AS [QuantityPerUnit], 
    [Extent1].[UnitPrice] AS [UnitPrice], 
    [Extent1].[UnitsInStock] AS [UnitsInStock], 
    [Extent1].[UnitsOnOrder] AS [UnitsOnOrder], 
    [Extent1].[ReorderLevel] AS [ReorderLevel], 
    [Extent1].[Discontinued] AS [Discontinued]
    FROM [dbo].[Products] AS [Extent1]
    WHERE [Extent1].[CategoryID] = @EntityKeyValue1
GO

-- Region Parameters
DECLARE @EntityKeyValue1 Int = 3
-- EndRegion
SELECT 
    [Extent1].[ProductID] AS [ProductID], 
    [Extent1].[ProductName] AS [ProductName], 
    [Extent1].[SupplierID] AS [SupplierID], 
    [Extent1].[CategoryID] AS [CategoryID], 
    [Extent1].[QuantityPerUnit] AS [QuantityPerUnit], 
    [Extent1].[UnitPrice] AS [UnitPrice], 
    [Extent1].[UnitsInStock] AS [UnitsInStock], 
    [Extent1].[UnitsOnOrder] AS [UnitsOnOrder], 
    [Extent1].[ReorderLevel] AS [ReorderLevel], 
    [Extent1].[Discontinued] AS [Discontinued]
    FROM [dbo].[Products] AS [Extent1]
    WHERE [Extent1].[CategoryID] = @EntityKeyValue1

2）渴望

SELECT 
    [Project1].[CategoryID] AS [CategoryID], 
    [Project1].[CategoryName] AS [CategoryName], 
    [Project1].[Description] AS [Description], 
    [Project1].[Picture] AS [Picture], 
    [Project1].[C1] AS [C1], 
    [Project1].[ProductID] AS [ProductID], 
    [Project1].[ProductName] AS [ProductName], 
    [Project1].[SupplierID] AS [SupplierID], 
    [Project1].[CategoryID1] AS [CategoryID1], 
    [Project1].[QuantityPerUnit] AS [QuantityPerUnit], 
    [Project1].[UnitPrice] AS [UnitPrice], 
    [Project1].[UnitsInStock] AS [UnitsInStock], 
    [Project1].[UnitsOnOrder] AS [UnitsOnOrder], 
    [Project1].[ReorderLevel] AS [ReorderLevel], 
    [Project1].[Discontinued] AS [Discontinued]
    FROM ( SELECT 
        [Limit1].[CategoryID] AS [CategoryID], 
        [Limit1].[CategoryName] AS [CategoryName], 
        [Limit1].[Description] AS [Description], 
        [Limit1].[Picture] AS [Picture], 
        [Extent2].[ProductID] AS [ProductID], 
        [Extent2].[ProductName] AS [ProductName], 
        [Extent2].[SupplierID] AS [SupplierID], 
        [Extent2].[CategoryID] AS [CategoryID1], 
        [Extent2].[QuantityPerUnit] AS [QuantityPerUnit], 
        [Extent2].[UnitPrice] AS [UnitPrice], 
        [Extent2].[UnitsInStock] AS [UnitsInStock], 
        [Extent2].[UnitsOnOrder] AS [UnitsOnOrder], 
        [Extent2].[ReorderLevel] AS [ReorderLevel], 
        [Extent2].[Discontinued] AS [Discontinued], 
        CASE WHEN ([Extent2].[ProductID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
        FROM   (SELECT TOP (3) [c].[CategoryID] AS [CategoryID], [c].[CategoryName] AS [CategoryName], [c].[Description] AS [Description], [c].[Picture] AS [Picture]
            FROM [dbo].[Categories] AS [c] ) AS [Limit1]
        LEFT OUTER JOIN [dbo].[Products] AS [Extent2] ON [Limit1].[CategoryID] = [Extent2].[CategoryID]
    )  AS [Project1]
    ORDER BY [Project1].[CategoryID] ASC, [Project1].[C1] ASC

3）显式

SELECT TOP (3) 
    [c].[CategoryID] AS [CategoryID], 
    [c].[CategoryName] AS [CategoryName], 
    [c].[Description] AS [Description], 
    [c].[Picture] AS [Picture]
    FROM [dbo].[Categories] AS [c]
GO

-- Region Parameters
DECLARE @EntityKeyValue1 Int = 1
-- EndRegion
SELECT 
    [Extent1].[ProductID] AS [ProductID], 
    [Extent1].[ProductName] AS [ProductName], 
    [Extent1].[SupplierID] AS [SupplierID], 
    [Extent1].[CategoryID] AS [CategoryID], 
    [Extent1].[QuantityPerUnit] AS [QuantityPerUnit], 
    [Extent1].[UnitPrice] AS [UnitPrice], 
    [Extent1].[UnitsInStock] AS [UnitsInStock], 
    [Extent1].[UnitsOnOrder] AS [UnitsOnOrder], 
    [Extent1].[ReorderLevel] AS [ReorderLevel], 
    [Extent1].[Discontinued] AS [Discontinued]
    FROM [dbo].[Products] AS [Extent1]
    WHERE [Extent1].[CategoryID] = @EntityKeyValue1
GO

-- Region Parameters
DECLARE @EntityKeyValue1 Int = 2
-- EndRegion
SELECT 
    [Extent1].[ProductID] AS [ProductID], 
    [Extent1].[ProductName] AS [ProductName], 
    [Extent1].[SupplierID] AS [SupplierID], 
    [Extent1].[CategoryID] AS [CategoryID], 
    [Extent1].[QuantityPerUnit] AS [QuantityPerUnit], 
    [Extent1].[UnitPrice] AS [UnitPrice], 
    [Extent1].[UnitsInStock] AS [UnitsInStock], 
    [Extent1].[UnitsOnOrder] AS [UnitsOnOrder], 
    [Extent1].[ReorderLevel] AS [ReorderLevel], 
    [Extent1].[Discontinued] AS [Discontinued]
    FROM [dbo].[Products] AS [Extent1]
    WHERE [Extent1].[CategoryID] = @EntityKeyValue1
GO

-- Region Parameters
DECLARE @EntityKeyValue1 Int = 3
-- EndRegion
SELECT 
    [Extent1].[ProductID] AS [ProductID], 
    [Extent1].[ProductName] AS [ProductName], 
    [Extent1].[SupplierID] AS [SupplierID], 
    [Extent1].[CategoryID] AS [CategoryID], 
    [Extent1].[QuantityPerUnit] AS [QuantityPerUnit], 
    [Extent1].[UnitPrice] AS [UnitPrice], 
    [Extent1].[UnitsInStock] AS [UnitsInStock], 
    [Extent1].[UnitsOnOrder] AS [UnitsOnOrder], 
    [Extent1].[ReorderLevel] AS [ReorderLevel], 
    [Extent1].[Discontinued] AS [Discontinued]
    FROM [dbo].[Products] AS [Extent1]
    WHERE [Extent1].[CategoryID] = @EntityKeyValue1

我已经完成了这个帖子，它涵盖了我对急切加载和延迟加载的疑问。但显式加载仍然有点混乱

Lazy Loading vs Eager Loading

Eager , Lazy and explicit loading in EF6

上面的文章真的帮助我解决了我的问题

只有一个疑问......为什么我要明确地加载一些东西...我明确加载它然后使用它（明确）..更好我在需要它时加载它（懒惰）..到这一点有没有使用显式加载..何时在延迟加载时使用显式加载？任何帮助

现在我的问题是， 1）什么时候用？特别是我们真的需要明确加载吗？如果是，我们什么时候需要呢？ 2）对我来说，为延迟加载和显式加载生成的查询几乎相似？懒惰和显式加载之间的实际区别是什么？