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

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);