How does the (u => u.Id == 123) lambda parse/execute? The bad way would be to load all users from the DB and filter on the server, the good way is to send a WHERE clause to the DB in the first place.

I ask because `Where(u => ComplicatedBooleanFunction(u))` can't in general be transformed into something you can do in SQL, especially if it's stateful, but maybe there's a trick if the function is simple enough?

Some ORMs get around this problem with methods like .WhereEqual(User::id, 123) so you don't need to parse anything to know the semantics.

PS. I have in the past got praised at work for massively speeding up database access, when I fixed something like `List<User> userList = UserDAO.fetchAll(); return userList.length();`. You need a minimal understanding of databases to use an ORM correctly. The DAO even had a `.count()` method, but you need to know a little about databases to know to look for that in the first place.