MongoDB covering index not working

Question

I have a countries document which looks like that:

{
  "_id" : ObjectId("4e493af4140700590800154f"),
  "geoname_id" : "49518",
  "code" : "rw",
  "names" : {
    "en" : "Rwanda",
    "nl" : "Rwanda",
    "de" : "Ruanda"
  }
}

In order to touch the index only when querying for:

db.countries.find({}, {"names.en":1, _id:0})

I added the following index:

db.countries.ensureIndex({"names.en":1})

To my understanding the query should now only touch the index. However, .explain() tells me that the query isnt using any index at all:

{
  "cursor" : "BasicCursor",
  "nscanned" : 247,
  "nscannedObjects" : 247,
  "n" : 247,
  "millis" : 0,
  "nYields" : 0,
  "nChunkSkips" : 0,
  "isMultiKey" : false,
  "indexOnly" : false,
  "indexBounds" : {
  }
}

I thought that the reason might be that the full db will be output (247 countries) but that doesnt make any sense to me. When the countries are available within the index the index should be used, right?

Anybody has an idea?

Cheers

Answer 1

The reason it's not using an index is because you're not querying on any criteria. Without find criteria the query optimizer will not select an index to use and therefor cannot function as a covered index.

Try db.countries.find({"names.en":"Rwanda"}, {"names.en":1, _id:0}).explain() to verify that with an indexed criteria it will in fact hit the appropriate index.

A case can be made for MongoDB needing to be smart enough to realize it can use the index to satisfy the original query, but currently it does not. You can use .sort({"names.en":1}) to make it select the index as well.

Answer 2

As of now, you can hint Mongo to use the index you want (ref):

db.countries.find({}, {"names.en":1, _id:0}).hint({"names.en":1})

I'm not sure when this feature was added, though.

Answer 3

Using a sort will ensure that it will use the index, FOR SORTING only.

Treat the "find" and the "sort" as two separated operations.