Mongodb – Index selection in MongoDB 3.0 for $or query

mongodbmongodb-3.0

I have a big $or query and MongoDB is not picking the index that I expect it to.

Query:

{"$or" : [
    {"del_at" : {"$gt" : Timestamp(1448895834, 2)}}, 
    {"rts" : {"$elemMatch" : {"k" : "app","v" : "38a91025-1109-4256-b08c-df559c0a74df","a" : false,"del_at" : {"$gt" : Timestamp(1448895834, 2)}}}}, 
    {"rts" : {"$elemMatch" : {"k" : "appv","v" : "e1d1daa4-8fb0-4aef-a6a2-9d80005bb8de","a" : false,"del_at" : {"$gt" : Timestamp(1448895834, 2)}}}}, 
    {"rts" : {"$elemMatch" : {"k" : "devg","v" : "ebfb77ea-3568-4725-84f6-8d5ee2582868","a" : false,"del_at" : {"$gt" : Timestamp(1448895834, 2)}}}}, 
    {"rts" : {"$elemMatch" : {"k" : "dev","v" : "5275cb0e-85f1-4f88-970b-e76ee0c55576","a" : false,"del_at" : {"$gt" : Timestamp(1448895834, 2)}}}}, 
    {"rts" : {"$elemMatch" : {"k" : "usrg","v" : "acab2d5a-2814-4e19-98a0-95b4e0748789","a" : false,"del_at" : {"$gt" : Timestamp(1448895834, 2)}}}}, 
    {"rts" : {"$elemMatch" : {"k" : "usr","v" : "d3f2aef1-64d4-4d3f-a36a-cdfb0d496c94","a" : false,"del_at" : {"$gt" : Timestamp(1448895834, 2)}}}}
]}

Indexes:

1. {"del_at" : 1,"rts.k" : 1,"rts.v" : 1,"rts.a" : 1,"rts.del_at" : 1}
2. {"rts.a" : 1,"rts.k" : 1,"rts.v" : 1,"del_at" : 1,"up_at" : 1}

In MongoDB 2.6, it does not select an index, however both exist.

In MongoDB 3.0 and 3.2, it selects the second index, but I'd expect it to use the first. Does anyone know why?

Thanks for the help!

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