Question

I installed the lastest MongoDB 64 bit DB and official C# driver as of 13 Marh 2012. I am getting some unexpected performace results with cursors.

The following code will retrieve and loop through 500,000 records at about 26.8 k / sec on my Core 2 Duo 2 GHz laptop:

    var query = Query.EQ("_H._t", "Car");
    var cursor = mc.FindAs<RoctObj>(query);
    double priceTot = 0d;

    foreach (RoctObj item in cursor)
    {
        Car car = (Car)item._H;
        priceTot += car.Price;
    }

That seems reasonable. Next, I adjusted the query so that only 721 results are returned. The code takes over 1.1 seconds longer to execute than if the foreach segment is replaced with:

    long i = cursor.Count();

Given the speed of the first example, 721 records should only take a fraction of a second to iterate. I know there are some other overheads, but they should be that bad. I don’t understand why I am getting +1.1 seconds.

Any ideas?

EDIT

Here is the alternate query. Note that the query time isn’t the question. It’s the iteration time.

    var query = Query.And(
        Query.LTE("_H.Price", BsonDouble.Create(80000d)).GTE(BsonDouble.Create(40000d)),
        Query.LTE("_H.Cylinders", BsonDouble.Create(8d)).GTE(BsonDouble.Create(4d)),
        Query.LTE("_H.Capacity", BsonDouble.Create(3000d)).GTE(BsonDouble.Create(2000d)),
        Query.LTE("_H.TopSpeed", BsonDouble.Create(200d)).GTE(BsonDouble.Create(100d))
        );

Answer 1

MongoDB does not return all the results at once, it returns a cursor which reads data off the database one record at a time, as your application asks for it (i.e. during your iterations) which may be why it is slower.

Running a count() simply returns the amount of matches found but without data.