> db.foo.insert({})
> db.foo.find(

{a:undefined}

)

{ "_id" : ObjectId("4bfd52c6deec96b27dbb35da") }

> db.foo.ensureIndex(

{a:1}

)
> db.foo.find(

{a:undefined}

)
> db.foo.find(

{a:undefined}

).explain()
{
"cursor" : "BtreeCursor a_1",
"indexBounds" : [
[

{ "a" : null }

,

{ "a" : null }

]
],
"nscanned" : 0,
"nscannedObjects" : 0,
"n" : 0,
"millis" : 0,
"oldPlan" : {
"cursor" : "BtreeCursor a_1",
"indexBounds" : [
[

{ "a" : null }

,

{ "a" : null }

]
]
},
"allPlans" : [
{
"cursor" : "BtreeCursor a_1",
"indexBounds" : [
[

{ "a" : null }

,

{ "a" : null }

]
]
}
]
}