testshard3:PRIMARY> db.coll.getIndexes()
[
	{
		"v" : 1,
		"key" : {
			"_id" : 1
		},
		"name" : "_id_",
		"ns" : "db.coll"
	},
	{
		"v" : 1,
		"key" : {
			"pid" : "hashed"
		},
		"name" : "pid_hashed",
		"ns" : "db.coll"
	},
	{
		"v" : 1,
		"key" : {
			"pid" : 1,
			"key" : 1
		},
		"name" : "pid_1_key_1",
		"ns" : "db.coll"
	},
	{
		"v" : 1,
		"key" : {
			"pid" : 1,
			"guid" : 1
		},
		"name" : "pid_1_guid_1",
		"ns" : "db.coll"
	}
]

>> pid is shard key