How to efficiently perform “distinct” with multiple keys?

Question

For example, there is a collection like this:

{market: \'SH\', code: \'000001\', date: \'2012-01-01\', price: 1000}
{market: \'SZ\', code:

Answer 1

If you are willing to wait for the upcoming 2.2 release of MongoDB, you can run this query efficiently using the aggregation framework:

collection = db.tb;
result = collection.aggregate( 
            [
                {"$group": { "_id": { market: "$market", code: "$code" } } }
            ]
        );
printjson(result);

On a million-record collection on my test machine, this ran in 4 seconds, while the map/reduce version took over a minute.