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

Update for MongoDB 3.2

3.2 introduced $sample to the aggregation pipeline.

There's also a good blog post on putting it into practice.

For older versions (previous answer)

This was actually a feature request: http://jira.mongodb.org/browse/SERVER-533 but it was filed under "Won't fix."

The cookbook has a very good recipe to select a random document out of a collection: http://cookbook.mongodb.org/patterns/random-attribute/

To paraphrase the recipe, you assign random numbers to your documents:

db.docs.save( { key : 1, ..., random : Math.random() } )

Then select a random document:

rand = Math.random()
result = db.docs.findOne( { key : 2, random : { $gte : rand } } )
if ( result == null ) {
  result = db.docs.findOne( { key : 2, random : { $lte : rand } } )
}

Querying with both $gte and $lte is necessary to find the document with a random number nearest rand.

And of course you'll want to index on the random field:

db.docs.ensureIndex( { key : 1, random :1 } )

If you're already querying against an index, simply drop it, append random: 1 to it, and add it again.