Why multiKey indexes blocks indexOnly queries in MongoDB?

Question

I am trying to use only the index while reading data from a collection in MongoDB, because I have some big documents, while for this query I need only one field.

It turns out that I cannot have indexOnly = true if the index is a multiKey index.

Here is the test I made:

db.test.drop()
db.test.insert({a:1})
db.test.ensureIndex({a:1})
db.test.find({a:1}, {_id:0, a:1}).explain()

-> indexOnly = true, isMultiKey = false

db.test.insert({a : [2,3]})
db.test.find({a:1}, {_id:0, a:1}).explain()

-> indexOnly = false, isMultiKey = true

The documentation mentions some limitations of the multikey indexes, but not this one. Does anybody have an idea how to use both multikey and indexonly?

Answer 1

From: http://docs.mongodb.org/manual/tutorial/create-indexes-to-support-queries/#create-indexes-that-support-covered-queries

An index cannot cover a query if any of the indexed fields in any of the documents in the collection includes an array. If an indexed field is an array, the index becomes a multi-key index index and cannot support a covered query.

You are inserting an array into your test collection, so when mongo is creating the index, it has to create a MultiKey index (it means it is creating the index for each item of the array).