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 there and there are no field in which I can generate a random number and obtain a random row. Any suggestions?

Answer 1

Do a count of all records, generate a random number between 0 and the count, and then do:

db.yourCollection.find().limit(-1).skip(yourRandomNumber).next(). 

Answer 2

it is tough if there is no data there to key off of. what are the _id field? are they mongodb object id's? If so, you could get the highest and lowest values:

lowest = db.coll.find().sort({_id:1}).limit(1).next()._id;
highest = db.coll.find().sort({_id:-1}).limit(1).next()._id;

then if you assume the id's are uniformly distributed (but they aren't, but at least it's a start):

unsigned long long L = first_8_bytes_of(lowest)
unsigned long long H = first_8_bytes_of(highest)

V = (H - L) * random_from_0_to_1();
N = L + V;
oid = N concat random_4_bytes();

randomobj = db.coll.find({_id:{$gte:oid}}).limit(1);

Answer 3

I'd suggest adding a random int field to each object. Then you can just do a findOne({random_field: {$gte: rand()}}) to pick a random document. Just make sure you ensureIndex({random_field:1})

Answer 4

The $gte: rand funtion does not work with that! $gte will return the FIRST value that is greater than the random number, not the number that is closest to the random number!