MongoDb c# driver LINQ vs Native query

Question

Which of these queries is better performance wise one uses linq and the other uses a native querying mehanism

LINQ

var query =
collection.AsQueryable<Employee>()
.Where(e => e.FirstName == "John")
.Select(e => e); 


NATIVE

var query= Query<Employee>.EQ(e => e.FirstName, "John");
var emp = collection.FindOne(query);

I am assuming that the native querying mechanism in mongoDb would be better since, as far as I understand, it filters out the result in the database where as linq first gets all the items in the collection and then filters for the result.When should I consider using LINQ over native querying mechanism?

Answer 1

since, as far as I understand, it filters out the result in the database where as linq first gets all the items in the collection and then filters for the result

No, no and no. It filter everything on a database level.

Second one is not native way to build mongodb query. It is still same wrapper just build real native mongod query(for example {"a": 1, "b": 2 }).

First query use expression trees to build mongo query. Second one use reflection to build same query. As I know expression trees should be faster (here is first article I found). Anyway, I don't think that performance difference essential. So just use what you like more.

As for me, I use linq for most queries and some developers may even don't know much about mongo, but black side here is that you actually may not realize what query it can build for you. (the same situation with linq to sql for example). But you still be able to log all native queries, review them and find places for optimization, changes. For the complex queries I usually use second approach.