Why multiKey indexes blocks indexOnly queries in MongoDB?
mongodb, multikey
Solution
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).
Problem
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?