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.

In EF Core this will almost certainly generate a query like "SELECT name FROM users WHERE id = 123 LIMIT 1;". The LINQ expression is transformed into SQL, and any expression that can't be transformed will cause an error to be thrown.

Certain methods like First() in this case materialize expressions. Only those trigger a DB query.