CODEWITHSUNDEEP
mongodb
techbech
techbech

Reputation: 3874

Is MongoDB _id unique by default?

Is MongoDB _id unique by default, or do I have to set it to unique?

Upvotes: 24

Views: 14863

Answers (5)

Max
Max

Reputation: 2836

For the most part, _id in mongodb is unique enough. There is one edge case where it's possible to generate a duplicate id though:

If you generate more than (2^(3 * 8))-1 = 16,777,215 object ids in the same single second, you could get a duplicate id due to how object ids are generated, otherwise you are alright. If you did generate that many ids in the same second, you are in a lottery and have a 1 in 2^(5 * 8) chance on top of this to get your duplicate id.

Let me know if you ever manage to pull this off with a realistic use case. Apparently google only gets around 70,000 searches a second.

Here is my source: https://www.mongodb.com/blog/post/generating-globally-unique-identifiers-for-use-with-mongodb

ObjectID is a 96-bit number which is composed as follows:

  • a 4-byte timestamp value representing the seconds since the Unix epoch (which will not run out of seconds until the year 2106)
  • a 5-byte random value, and
  • a 3-byte incrementing counter, starting with a random value.

Update: I have also heard the 5-byte random value could instead be replaced by 3 bytes representing the machine id and 2 bytes representing the process id.

Upvotes: 28

Chris Bao
Chris Bao

Reputation: 2888

Share what I learned about this topic here:

Different from other RDBMS, Mongodb document's Id is generated on the client side. This functionality is usually implemented in the drivers of various programming languages.

The Id is string with 12 bytes length, which consists of several parts as follows:

TimeStamp(4 bytes) + MachineId(3 bytes) + ProcessId(2 bytes) + Counter(3 bytes)

Based on this pattern, it's extremely unlikely to have two Ids duplicated.

Upvotes: 0

EliadL
EliadL

Reputation: 7088

According to MongoDB's manual the answer is yes, it's unique by default:

MongoDB creates the _id index, which is an ascending unique index on the _id field, for all collections when the collection is created. You cannot remove the index on the _id field.

Upvotes: 3

Baba
Baba

Reputation: 95161

ok .. short version

YES YES YES

_id uniqid by default , mongoDB creates index on _id by default and you do not need any settings

Upvotes: 4

Sergio Tulentsev
Sergio Tulentsev

Reputation: 230541

All documents contain an _id field. All collections (except for capped ones) automatically create unique index on _id.

Try this:

db.system.indexes.find()

Upvotes: 14

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