Template
single collection
multiple collections
mgodatagen
update
indexe
explain
Stage
Database
bson
mgodatagen
[{"a":"a"},{"a":"a"},{"a":"a"},{"a":"a"},{"a":"a"},{"a":"a"}]
Query
db.collection.aggregate([{"$group":{"_id":null,"ids":{"$addToSet":"$_id"}}},{"$project":{"_id":0}}])
Result