LINQ to SQL join生成连接IS NULL的SQL

我不擅长Linq的表达,今天我遇到了一个奇怪的问题,如下面的内连接语句,

var orders = (from q in dao.CurrentDBContext.New_OrderForm 
              join d in dao.CurrentDBContext.New_OrderGoodsDetail on q.billNum equals d.billNum
              select new
             {    
                q.billNum,
                q.orderSource,
                q.sourceOddNum    
                d.PPT    
             }

当我追踪linq语句时,我很困惑,Entity Framework将linq语句转换为下面的sql语句

SELECT 
[Extent1].[billNum] AS [billNum], 
[Extent1].[orderSource] AS [orderSource], 
[Extent1].[sourceOddNum] AS [sourceOddNum], 
[Extent2].[PPT] AS [PPT]
FROM [dbo].[New_OrderForm] AS [Extent1]
INNER JOIN [dbo].[New_OrderGoodsDetail] AS [Extent2] 
           ON ([Extent1].[billNum] = [Extent2].[billNum]) OR 
              (([Extent1].[billNum] IS NULL) AND ([Extent2].[billNum] IS NULL))

你知道为什么下面的SQL段会自动附加吗？

或(([Extent1].[billNum] IS NULL)AND([Extent2].[billNum] IS NULL) "

我不希望上面的内容自动附加,因为它确实降低了SQL性能.有什么建议？