Random record from MongoDB

Question

I am looking to get a random record from a huge (100 million record) mongodb.

What is the fastest and most efficient way to do so? The data is already t

Answer 1

The following recipe is a little slower than the mongo cookbook solution (add a random key on every document), but returns more evenly distributed random documents. It's a little less-evenly distributed than the skip( random ) solution, but much faster and more fail-safe in case documents are removed.

function draw(collection, query) {
    // query: mongodb query object (optional)
    var query = query || { };
    query['random'] = { $lte: Math.random() };
    var cur = collection.find(query).sort({ rand: -1 });
    if (! cur.hasNext()) {
        delete query.random;
        cur = collection.find(query).sort({ rand: -1 });
    }
    var doc = cur.next();
    doc.random = Math.random();
    collection.update({ _id: doc._id }, doc);
    return doc;
}

It also requires you to add a random "random" field to your documents so don't forget to add this when you create them : you may need to initialize your collection as shown by Geoffrey

function addRandom(collection) { 
    collection.find().forEach(function (obj) {
        obj.random = Math.random();
        collection.save(obj);
    }); 
} 
db.eval(addRandom, db.things);

Benchmark results

This method is much faster than the skip() method (of ceejayoz) and generates more uniformly random documents than the "cookbook" method reported by Michael:

For a collection with 1,000,000 elements:

• This method takes less than a millisecond on my machine

• the skip() method takes 180 ms on average

The cookbook method will cause large numbers of documents to never get picked because their random number does not favor them.

• This method will pick all elements evenly over time.

• In my benchmark it was only 30% slower than the cookbook method.

• the randomness is not 100% perfect but it is very good (and it can be improved if necessary)

This recipe is not perfect - the perfect solution would be a built-in feature as others have noted.
However it should be a good compromise for many purposes.