EF 4.1: Why does turning a constant into a variable result in extra sub query?

Question

Today I discovered that Entity Framework was adding an unnecessary sub query to the SQL it generates. I started digging my code trying to narrow down where it might come fro

Answer 1

Like people said. Difference between both queries is minimal.

The reason is that the expression, that is created when you create your LINQ is different when you use variable and when constant. And EF will catch this and will generate your SQL acordingly. It knows it will never change, so it can be hard-coded into the query for (possible) performance gain.

Edit: I dont think there is answer to this question except "Thats how EF does it." But it is very well known that EF likes to creates many subselects. It can result for many subselects for more complex queries. Some even dismis even using EF for this fact. But this is simply price for using tool like EF. You loose fine-grained control over something, that can have big performance gain. Why do you use .NET, when you can use C and gain more performance? Why use C when you can use assembly, to gain more performance gain?

Only way to be safe and still be able to use high-abstraction layer EF is to use SQL profiller often and check if there are not queries that take too long on real data. And if your find some then either conver them to direct SQL or stored procedures.