Simple MongoDB query very slow although index is set

Question

I\'ve got a MongoDB collection that holds about 100M documents.

The documents basically look like this:

_id             : ObjectId(\"asd1234567890\")
_re         


        

Answer 1

I would try setting a non-unique index on _reference_2_id, because at the moment, I suspect you'll be doing the equivalent of a full table scan as even though the indexes contain _reference_2_id, they won't be used (see here).