> db.foo.ensureIndex({a:1})
> db.foo.ensureIndex({b:1})
> db.foo.insert({a:1})
> db.foo.insert({b:1})
> db.foo.insert({a:1, b:1})
> db.foo.find({$or:[{a:1}, {b:1}]}).explain()
{
	"clauses" : [
		{
			"cursor" : "BtreeCursor a_1",
			"nscanned" : 2,
			"nscannedObjects" : 2,
			"n" : 2,
			"millis" : 0,
			"nYields" : 0,
			"nChunkSkips" : 0,
			"isMultiKey" : false,
			"indexOnly" : false,
			"indexBounds" : {
				"a" : [
					[
						1,
						1
					]
				]
			},
			"allPlans" : [
				{
					"cursor" : "BtreeCursor a_1",
					"indexBounds" : {
						"a" : [
							[
								1,
								1
							]
						]
					}
				}
			]
		},
		{
			"cursor" : "BtreeCursor b_1",
			"nscanned" : 2,
			"nscannedObjects" : 2,
			"n" : 1,
			"millis" : 0,
			"nYields" : 0,
			"nChunkSkips" : 0,
			"isMultiKey" : false,
			"indexOnly" : false,
			"indexBounds" : {
				"b" : [
					[
						1,
						1
					]
				]
			},
			"allPlans" : [
				{
					"cursor" : "BtreeCursor b_1",
					"indexBounds" : {
						"b" : [
							[
								1,
								1
							]
						]
					}
				}
			]
		}
	],
	"nscanned" : 4,
	"nscannedObjects" : 4,
	"n" : 3,
	"millis" : 0
}