Mongo > db.test1.insert({a : "very large string......"})
Mongo > db.test1.findOne({a : "very large string......"})
{ "_id" : ObjectId("519bb718ca8a4b212b90b17e"), "a" : "very large string......" }

Mongo > db.test1.ensureIndex({a : 1})
Mongo > db.test1.findOne({a : "very large string......"})
null