i have a mongodb db with more than 100 millions documents. i want to do aggregation so i can give statistics on documents. my document look like :

{
    "categ": "categ_4", 
    "code": 200, 
    "date": "01/01/2017", 
    "host": "www.myhost.com", 
    "hour": "19", 
    "http_ver": "HTTP/1.1", 
    "idate": 20170101, 
    "length": 21, 
    "protocol": "https", 
    "remote_ip": "111.22.333.44", 
    "resp_time": 0, 
    "time": "19:53:15", 
    "url": "my_url", 
}

when aggregating, i perform a query like this in my shell :

db.data.aggregate([{"$match": {"code":200}}, {"$group": {_id : "$code", total : {"$sum" : 1}}},{"$sort" : {_id: 1}}])

the problem is that it takes a very long time to compute. this is too slow. is there any way to speed up this operation ? i tryed to create index on "code" field but with no success

db.data.createIndex({code:1})

what can I do to make aggregation faster ?

thank you