Question

Can you perform a MongoDB covered query for two fields, for example

db.collection.find( { _id: 1, a: 2 } )

without having a compound index such as

db.collection.ensureIndex( { _id: 1, a: 1 } )

but instead having only one index for _id (you get that by default) and another index for field “a”, as in

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

In other words, I’d like to know if in order to perform a covered query for two fields I need a compound index vs. needing only two single (i.e., not compound) indexes, one for each field.

Answer 1

Queries only use one index.

Your example shows _id as one of the elements of your index? _id Needs to be unique in a collection, so it wouldn’t make sense to make a compound index of _id and something else.

If you instead had:

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

You could then use the a index as needed, independently, or as a compound index with b.