MongoDB indexes issue

Question

MongoDB can store documents with different fields in one collection. How then indexes will work? If I create index on field that presents not in all documents, the documents which don't have that will not be indexed?

Answer 1

Documents without the field in an index will be indexed as having no value for that field. You probably want to review this: http://docs.mongodb.org/manual/core/indexes/

If you want to not include documents that don't have the key in the index, you can use a sparse index: http://docs.mongodb.org/manual/administration/indexes/#sparse-indexes