Cardinality	Write scaling	Query isolation	Index Locality
_id	Doc level	|ff0000|1 shard|	|ff0000|All shards, merge sort|	Great
hash(_id)	Hash level	All shards	|ff0000|All shards, merge sort|	|ff0000|Poor|
user	|ff0000|Many docs|	All shards	One shard, index sort	So-so
user,time	Doc level	All shards	One shard, index sort	Good

{ _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: "..." }