Index selection in MongoDB 3.0 for $or query

Question

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!

Answer 1

I asked this question on the Google group for Mongodb. See Dwight's explanation and work around.

https://groups.google.com/forum/?__hstc=21977188.535555756f91d668db0d32d8604f6233.1443211966594.1443211966594.1443211966594.1&__hssc=21977188.11.1443211966594&__hsfp=469890013#!topic/mongodb-user/iKtaqA5KrKY

Thanks!