I'm a MongoDB noob and I have a MongoDB collection with millions of documents, each with tens of keys (average ~ 60, large variance) from a set of around 100. I'd like to get a count of each key across the entire collection. For example, if the documents in the collection were: ```json {"_id":0, "foo": 0, "bar":1, "baz":2} {"_id":1, "foo": 0, "baz":7, "qux":11, "quux":13} {"_id":2, "foo": 1, "bar":1, "quux":3} ``` then the desired output would be: ```json {"_id":3, "foo":3, "bar":2, "baz":2, "qux":1, "quux": 2} ``` I can "explode" the collection with `$objectToArray`, `$unwind`, `$group`, and then `$count`, but it's slow. Is there something that could do this efficiently in one pass through the collection? Something like, ``` [notional psuedocode] output={} foreach document: foreach key: if output.key exists: output.key+=1 else: output.key=1 => output: {key1: key1_count, ...} ```