> db.tmp.find()
{ "_id" : ObjectId("4efc37bb80f98024da20613f"), "a" : 10, "c" : 2 }
{ "_id" : ObjectId("4efc37c180f98024da206140"), "a" : 11, "c" : 23 }
{ "_id" : ObjectId("4efc37c480f98024da206141"), "a" : 11 }
{ "_id" : ObjectId("4efc37cd80f98024da206142"), "a" : "Test" }
{ "_id" : ObjectId("4efc38c880f98024da206143"), "a" : "Test", "c" : 1231 }
>db.tmp.ensureIndex({c:1},{sparse:true})
>db.tmp.find({c:{$exists:true}}).explain()
{
	"cursor" : "BasicCursor",
	"nscanned" : 5,
	"nscannedObjects" : 5,
	"n" : 3,
	"millis" : 0,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"isMultiKey" : false,
	"indexOnly" : false,
	"indexBounds" : {
		
	}
}
> db.tmp.find({c:{$exists:false}}).explain()
{
	"cursor" : "BtreeCursor c_1",
	"nscanned" : 0,
	"nscannedObjects" : 0,
	"n" : 0,
	"millis" : 0,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"isMultiKey" : false,
	"indexOnly" : false,
	"indexBounds" : {
		"c" : [
			[
				null,
				null
			]
		]
	}
}