LINQ To Enties问题

[sz_free_bird]

using (NorthwindEntities context = new NorthwindEntities())
            {
                Expression<Func<NorthwindEFDemo.Product, bool>> expr = p =>
                        (String.IsNullOrEmpty(textBox1.Text) ? true : p.ProductName.Contains(textBox1.Text));
                var data = context.Products.Where(expr.Compile()).Select(p => new
                {
                    PorudctName= p.ProductName,
                    CategoryName=p.Category.CategoryName
                });
                dataGridView1.DataSource = data.ToList();
            }

上面的代码使用Northwind的示例数据库。目的是想实现一个动态查询。功能基本没问题。但有疑问。就是生成SQL脚本的问题。加入在textBox1中输入的：ab。下面贴上生成的脚本

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]

exec sp_executesql N'SELECT 
[Extent1].[CategoryID] AS [CategoryID], 
[Extent1].[CategoryName] AS [CategoryName], 
[Extent1].[Description] AS [Description], 
[Extent1].[Picture] AS [Picture]
FROM [dbo].[Categories] AS [Extent1]
WHERE [Extent1].[CategoryID] = @EntityKeyValue1',N'@EntityKeyValue1 int',@EntityKeyValue1=4

我的疑问在，为什么在Product的过滤语句中没有where子句的生成。
如果我换一种写法，不使用表达式树。代码如下

 using (NorthwindEntities context = new NorthwindEntities())
            {
                //Expression<Func<NorthwindEFDemo.Product, bool>> expr = p =>
                //        (String.IsNullOrEmpty(textBox1.Text) ? true : p.ProductName.Contains(textBox1.Text));
                var data = context.Products.Where(p=>p.ProductName.Contains(textBox1.Text)).Select(p => new
                {
                    PorudctName= p.ProductName,
                    CategoryName=p.Category.CategoryName
                });
                dataGridView1.DataSource = data.ToList();
            }

生成的SQL语句如下：

exec sp_executesql N'SELECT 
1 AS [C1], 
[Extent1].[ProductName] AS [ProductName], 
[Extent2].[CategoryName] AS [CategoryName]
FROM  [dbo].[Products] AS [Extent1]
LEFT OUTER JOIN [dbo].[Categories] AS [Extent2] ON [Extent1].[CategoryID] = [Extent2].[CategoryID]
WHERE [Extent1].[ProductName] LIKE @p__linq__0 ESCAPE N''~''',N'@p__linq__0 nvarchar(4000)',@p__linq__0=N'%ab%'

我想知道为什么这两种写法导致生成的SQL语句会有这么大的差别。而且第一种方式，根据生成的SQL来看，是整张表的数据都加载了，如果数据量大的话，性能是个很大的问题。

[边城的刀声]

貌似是expr.Compile()的问题
Compile的问题

[sz_free_bird]

为什么这么说呢？