How fast is it to look up by ObjectId in Mongodb?

Question

I'm looking up ObjectIds everywhere, as if they're cake.

Is this OK? _id fields are supposed to be looked up like crazy, right?

Sleuth

Surely this question is only meaningful if the look-up speed is relative to some other type of look-up, like an index on a string field, or an index on a unique string or integer field, or a non-indexed field? – Priddy

Related: #18680962 – Priddy

Answer 1

_id is the primary key. It's indexed. Of course it's fast.

Beaulahbeaulieu

Why do you say 'primary' key? I don't believe there are primary and foriegn keys in Mongo, just indexes. – Traweek

Technically, yes. From the glossary: "The _id field must have a unique value. You can think of the _id field as the document’s primary key." – Beaulahbeaulieu

Answer 2

A more precise answer: MongoDB uses B-Tree indexes. Searching for a particular value in a B-Tree has O(log n) complexity in the average and worst case, which can be considered reasonably fast (i.e. a binary search). It is not constant complexity = O(1) though, so you still might have some slowdown effects if the index size grows larger than available RAM. (MongoDB tries to keep the indexes in RAM, and every IO needed to look up an index on disk will slow down your query considerably).

Manisa

There is nothing like "constant complexity". The overall complexity is O(1). Point. – Outhouse

true, the word "time" was missing after "constant". – Manisa

Answer 3

ObjectIds, if your primary method of data access, will be the fastest way to retrieve your stuff from MongoDB. We utilize our MongoDB as a keyed repository for most of our data access. You'll have great results doing what you're doing.

Haynes

Answer 4

Index on _id field is auto created by mongo and default primary key. Speedwise, it will be super fast to access docs by _id field.

What concerns do you have?

Cameroun