Index hint with mongodb csharp

Question

I am migrating from the mongodb csharp driver 1.10.0 to 2.0.0. One of the collection I am using is very big and has to fulfill many queries with different filter attributes. That is why I was relying on some index hint statements. With the v1.10 driver it looks like

 myCollection.Find(query).SetHint("myIndexName");

I searched the v2 driver api but this hint method seems to be completly removed in the v2 driver. Is there an alternative? How should I do index hints with the v2 driver?

Note: The Solutions provided works for latest mongodb csharp drivers as well

Answer 1

You can use the FindOptions.Modifiers property.

var modifiers = new BsonDocument("$hint", "myIndexName"); 
await myCollection.Find(query, new FindOptions { Modifiers = modifiers }).ToListAsync();

May I ask why you are using the hint? Was the server consistently choosing the wrong index? You shouldn't need to do this except in exceptional cases.

Craig

Answer 2

Ideally, try to make the query in a way that mongodb optimizer can use the index automatically.

If you are using FindAsync then you will have a property named Hint. Use it like this:

If you have index named "myIndexName" which you want your query should use forcefully, then use like this:.

BsonString bsonString = new BsonString("myIndexName");

cursor = await collection.FindAsync(y => y.Population > 400000000,
new FindOptions<Person, Person>()
{
     BatchSize = 200,
     NoCursorTimeout = true,
     AllowPartialResults = true,
     Projection = "{'_id':1,'Name':1,'Population':1}"
     Hint = bsonString.AsBsonValue,
}).ConfigureAwait(false);

You can fine BsonString class in MongoDB.Bson