When writing .NET code to access a SQL database we often rely on the Entity Framework (EF). The EF makes it very easy to retrieve data from the database by generating a SQL Query for us. But we should not trust it blindly, as the EF can also generate a bad query. It will return the correct data yes, but at what performance cost?

With the introduction of LINQ the difference between writing code for accessing a lists of objects in memory and accessing a list of data in an external data source like SQL is vanishing. Combining a in memory with a external list in a single query was not yet possible. With the introduction of .NET Framework 4.0 this has changed.