> db.foo.drop()
true
> db.foo.ensureIndex({a:1})
{
	"createdCollectionAutomatically" : true,
	"numIndexesBefore" : 1,
	"numIndexesAfter" : 2,
	"ok" : 1
}
> db.foo.ensureIndex({b:1})
{
	"createdCollectionAutomatically" : false,
	"numIndexesBefore" : 2,
	"numIndexesAfter" : 3,
	"ok" : 1
}
> db.foo.insert({a:1,b:2})
WriteResult({ "nInserted" : 1 })
> db.foo.insert({a:1,b:2})
WriteResult({ "nInserted" : 1 })
> db.foo.insert({a:1,b:2})
WriteResult({ "nInserted" : 1 })
> db.foo.aggregate([{$match: {a:1,b:1}}],{explain:true})
{
	"stages" : [
		{
			"$cursor" : {
				"query" : {
					"a" : 1,
					"b" : 1
				},
				"planError" : "InternalError No plan available to provide stats"
			}
		}
	],
	"ok" : 1
}
>