Suggest an editImprove this articleRefine the answer for “What index is created automatically in MongoDB?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)By default, MongoDB automatically creates an index on the `_id` field: it's unique (a collection can't have two documents with the same `_id`), it's created together with the collection, and it's used for fast lookups, updates, and deletes on that field. **Key point:** even without explicitly creating any indexes, a collection always has `{ _id: 1 }`, a B-tree index that provides fast access to a document by its identifier.Shown above the full answer for quick recall.Answer (EN)ImageBy 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.For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.