Mongodb : $in operator vs lot of single queries

Question

I know MongoDB is able to handle a lot of requests/sec, but let's say I have to query a lot of documents of a collection given their _id; what sounds better: making a $in on the _id attribute with all the ids I want to get, or loop over findOne queries?

Answer 1

I would definitely go with using the $in query and providing a array of _ids.

Example:

db.collection.find({
    "key": {
        "$in": [
            ObjectId("xxx"),
            ObjectId("yyy"),
            ObjectId("zzz")
        ]
    }
})

Why?

• If you loop, there is a certain amount of setup and teardown for each query creating and exhausting cursors which would create overhead.
• If you are not doing this on a local machine it also creates tcp/ip overhead for every request. Locally you could use domain sockets.
• There is a index on "_id" created by default and collecting a group of documents to return in a batch request should be extremely fast so there is no need to break this up into smaller queries.

There's some additional documentation here if you want to check it out.