gistfile1.js

mongos> version()
version: 2.0.4

mongos> db.collections.find()
{ "_id" : "test.testCollection", "lastmod" : ISODate("1970-01-16T11:14:38.015Z"), "dropped" : false, "key" : { "shardkey" : 1 }, "unique" : false }

mongos> db.testCollection.find({_id:111}).explain()
{
	"clusteredType" : "ParallelSort",
	"shards" : {
		"brick1:27000" : [
			{
				"cursor" : "BtreeCursor _id_",
				"nscanned" : 0,
				"nscannedObjects" : 0,
				"n" : 0,
				"millis" : 0,
				"nYields" : 0,
				"nChunkSkips" : 0,
				"isMultiKey" : false,
				"indexOnly" : false,
				"indexBounds" : {
					"_id" : [
						[
							111,
							111
						]
					]
				}
			}
		],
		"brick1:27001" : [
			{
				"cursor" : "BtreeCursor _id_",
				"nscanned" : 0,
				"nscannedObjects" : 0,
				"n" : 0,
				"millis" : 0,
				"nYields" : 0,
				"nChunkSkips" : 0,
				"isMultiKey" : false,
				"indexOnly" : false,
				"indexBounds" : {
					"_id" : [
						[
							111,
							111
						]
					]
				}
			}
		],
		"brick2:27000" : [
			{
				"cursor" : "BtreeCursor _id_",
				"nscanned" : 0,
				"nscannedObjects" : 0,
				"n" : 0,
				"millis" : 0,
				"nYields" : 0,
				"nChunkSkips" : 0,
				"isMultiKey" : false,
				"indexOnly" : false,
				"indexBounds" : {
					"_id" : [
						[
							111,
							111
						]
					]
				}
			}
		],
		"brick2:27001" : [
			{
				"cursor" : "BtreeCursor _id_",
				"nscanned" : 0,
				"nscannedObjects" : 0,
				"n" : 0,
				"millis" : 0,
				"nYields" : 0,
				"nChunkSkips" : 0,
				"isMultiKey" : false,
				"indexOnly" : false,
				"indexBounds" : {
					"_id" : [
						[
							111,
							111
						]
					]
				}
			}
		],
		"brick3:27000" : [
			{
				"cursor" : "BtreeCursor _id_",
				"nscanned" : 0,
				"nscannedObjects" : 0,
				"n" : 0,
				"millis" : 0,
				"nYields" : 0,
				"nChunkSkips" : 0,
				"isMultiKey" : false,
				"indexOnly" : false,
				"indexBounds" : {
					"_id" : [
						[
							111,
							111
						]
					]
				}
			}
		],
		"brick3:27001" : [
			{
				"cursor" : "BtreeCursor _id_",
				"nscanned" : 0,
				"nscannedObjects" : 0,
				"n" : 0,
				"millis" : 0,
				"nYields" : 0,
				"nChunkSkips" : 0,
				"isMultiKey" : false,
				"indexOnly" : false,
				"indexBounds" : {
					"_id" : [
						[
							111,
							111
						]
					]
				}
			}
		]
	},
	"n" : 0,
	"nChunkSkips" : 0,
	"nYields" : 0,
	"nscanned" : 0,
	"nscannedObjects" : 0,
	"millisTotal" : 0,
	"millisAvg" : 0,
	"numQueries" : 6,
	"numShards" : 6
}

mongos> db.testCollection.find({shardkey:111}).explain()
{
	"clusteredType" : "ParallelSort",
	"shards" : {
		"brick1:27000" : [
			{
				"cursor" : "BtreeCursor shardkey_1",
				"nscanned" : 0,
				"nscannedObjects" : 0,
				"n" : 0,
				"millis" : 0,
				"nYields" : 0,
				"nChunkSkips" : 0,
				"isMultiKey" : false,
				"indexOnly" : false,
				"indexBounds" : {
					"shardkey" : [
						[
							111,
							111
						]
					]
				}
			}
		]
	},
	"n" : 0,
	"nChunkSkips" : 0,
	"nYields" : 0,
	"nscanned" : 0,
	"nscannedObjects" : 0,
	"millisTotal" : 0,
	"millisAvg" : 0,
	"numQueries" : 1,
	"numShards" : 1
}


mongos> db.testCollection.find({shardkey:111,_id:999}).explain()
{
	"clusteredType" : "ParallelSort",
	"shards" : {
		"brick1:27000" : [
			{
				"cursor" : "BtreeCursor _id_",
				"nscanned" : 0,
				"nscannedObjects" : 0,
				"n" : 0,
				"millis" : 0,
				"nYields" : 0,
				"nChunkSkips" : 0,
				"isMultiKey" : false,
				"indexOnly" : false,
				"indexBounds" : {
					"_id" : [
						[
							999,
							999
						]
					]
				}
			}
		]
	},
	"n" : 0,
	"nChunkSkips" : 0,
	"nYields" : 0,
	"nscanned" : 0,
	"nscannedObjects" : 0,
	"millisTotal" : 0,
	"millisAvg" : 0,
	"numQueries" : 1,
	"numShards" : 1
}