Mongo: Quick way to find the last n inserts into a collection
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I am running a query like the following:

    db.sales.find({location: "LLLA", created_at: {$gt: ISODate('2012-07-27T00:00:00Z')}}).sort({created_at: -1}).limit(3)

This query is working as expected, but its not performing fast enough. I have an index on the location field, and a separate index on the created_at field. Please let me know if you see me missing something obvious here to make it faster.

Thanks for your time on this.
