What is the _id field and ObjectId in MongoDB?

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Every MongoDB document has a unique _id field that serves as its primary key . If you don't provide one, MongoDB generates an ObjectId automatically. ObjectId — 12-byte value: 4-byte timestamp + 5-byte random + 3-byte counter. Roughly sortable by creation time. Indexed automatically — unique index on _id exists from the start; cannot be dropped. Custom values allowed — supply any unique value (string, integer, UUID) instead of ObjectId. Embeds the timestamp — ObjectId.getTimestamp() returns the creation moment without a separate column. Immutable — once set, can't be updated. Know the ObjectId structure: 4-byte timestamp + 5-byte random + 3-byte counter = 12 bytes. getTimestamp() extracts creation time. _id can be any type (not just ObjectId). _id is automatically indexed and unique. No need for auto-increment — ObjectId handles uniqueness globally.