MongoDB - too much data for sort() with no index error

Question

I am using MongoDB 1.6.3, to store a big collection (300k+ records). I added a composite index.

db['collection_name'].getIndexes()
[
    {
        "name" : "_id_",
        "ns" : "db_name.event_logs",
        "key" : {
            "_id" : 1
        }
    },
    {
        "key" : {
            "updated_at.t" : -1,
            "community_id" : 1
        },
        "ns" : "db_name.event_logs",
        "background" : true,
        "name" : "updated_at.t_-1_community_id_1"
    }
]

However, when I try to run this code:

db['collection_name']
  .find({:community_id => 1})
  .sort(['updated_at.t', -1])
  .skip(@skip)
  .limit(@limit)

I am getting:

Mongo::OperationFailure (too much data for sort() with no index. add an index or specify a smaller limit)

What am I doing wrong?

Answer 1

Try adding {community_id: 1, 'updated_at.t': -1} index. It needs to search by community_id first and then sort.