I'm still not over how LINQ is defined as a thin wrapper over both IQueryable (which can be efficiently queried) and IEnumerable (which can only be iterated), but IQueryable extends IEnumerable, so if you execute one unserializable operation anywhere in a query issued to your database it'll merrily pump whatever temporary collection happens to reside on the boundary through the C# program to execute that call on each row and process the rest as an IEnumerable.