What index is created automatically in MongoDB?

By default, MongoDB automatically creates an index on the _id field.

This index:

• is unique (a collection can't have two documents with the same _id)
• is created together with the collection
• is used for fast lookups by _id, as well as when updating and deleting documents by that field

So, even if you never explicitly create any indexes, a collection always has { _id: 1 }, a B-tree index that provides fast access to a document by its identifier.