Mongodb : $in operator vs lot of single queries

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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.

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