{ _id: ObjectId("51156a11056d6f966f268f7f"), user: 37113, time: ISODate( '2013-04-24 10:38:05 '), subject: "This is a test", recipients: [ "derick@10gen.com", "derick@example.com" ], body: "..." } Lots of emails per user Most common query: get user emails sorted by time Index on %{ _id: 1 }%, %{ user: 1, time: -1 }%,
and %{ recipients: 1 }%