Why I lose performance if I use LINQ on MongoDB?

Question

This is the situation. I have a Domain object Product like this...

[DataContract]
public class Product : IStorableEntity
{
    [DataMember]
    public String Id { get; set; }
    [DataMember]
    public String RemoteId { get; set; }
    [DataMember]
    public String LanguageId { get; set; }
    [DataMember]
    public DateTime? CreationDate { get; set; }
    [DataMember]
    public DateTime? LastUpdate { get; set; }
    ETC..ETC...
}

into my repository layer I have the following method.

public IEnumerable<TElement> Read()
{
    var mongoCollection = _mongoDatabase.GetCollection<TElement>(_partitionName);
    return mongoCollection.AsQueryable<TElement>();

}

With this method I want to expose via LINQ my repository layer without exporting information about technology.

In this way I can do this:

var _repository = new MyRepositoryFactory<Product>(); 
var result = _repository.Read().Where(p=>p.RemoteId == "1")

this query it takes 1 or 2 milliseconds.

instead...

var _repository = new MyRepositoryFactory<Product>();
var result = _repository.Read().Where(p=>p.RemoteId == "29000")

it takes 2800 milliseconds!!!

I have correctly created a unique index with the command

db.products.ensureIndex({"RemoteId":1, unique:true})

NB: Yes, I have also rebuilt the indexes with .reIndex() command

Here the strange thing... Avoiding LINQ and modifying the repository method in...

public IEnumerable<TElement> Read(string remoteId)
{
    var mongoCollection = _mongoDatabase.GetCollection<TElement>(_partitionName);
      var query = Query<TElement>.EQ(p => p.RemoteId, remoteId);
    return mongoCollection.Find(query);
}

if then I invoke the method whit the same id before..

var _repository = new MyMongoRepository<Product>();
var result = _repository.Read("29000")

it takes 1 or 2 milliseconds. WHY??

Why with the first approach do I have a performance degradation as the id increases instead with the second is not it?

Ps. Erm... really sorry for my english

Answer 1

As WiredPrainie stated in comments you should use IQueryable instead of IEnumerable otherwise the whole collection will be retrieved. Read this guide carefully http://docs.mongodb.org/ecosystem/tutorial/use-linq-queries-with-csharp-driver/