Question

What happens when you query two fields, given that they’re not part of a compound index, for example:

db.collection.ensureIndex( { a: 1 } )
db.collection.ensureIndex( { b: 1 } )

db.collection.find( { a: 2, b: 2 } )

I’d like to know if a number of documents might be scanned, or if Mongo DB won’t scan anything and the explain() will return:

indexOnly: YES

Answer 1

The explain only returns indexOnly if all the values you are looking for are in index (including the sort). Basically it denotes a covered index: http://docs.mongodb.org/manual/applications/indexes/#create-indexes-that-support-covered-queries

In this case you are querying by two fields but only one of them is in one individual index.

Considering MongoDB cannot use multiple indexes for a single clause ($or being the exception since it is multi-clause here also it is different than a normal query) it will not do an indexOnly query, instead it will search the collection by the index on a and then do a full document scan of the documents that contain b within that range to understand its values and return a result.

Do also note that to use indexOnly cursors properly you should make the following parts of the your query fit into a single index:

• Condition
• Projection
• Sort

i.e.:

db.ensureIndex({a:1,b:1});
db.col.find({a:1}, {_id:0,a:1,b:1}).sort({b:1});

Would use a indexOnly cursor.