Mongodb scanAndOrder index unexpected behaviour

Mongodb scanAndOrder index unexpected behaviour

I'm not understanding this mongodb behaviour, my collection has the
following index (names are simplified for clarity sake) and consists of ~
50k documents.
{
"v" : 1,
"key" : {
"a" : 1,
"b" : -1
},
"ns" : "db.articles",
"name" : "a_1_b_-1",
"background" : false,
"dropDups" : false
}
The following query
db.articles.find({ a: {"$in": ["foo", "bar"] } }).sort({b:
-1}).limit(10).explain()
returns:
{
"cursor" : "BtreeCursor a_1_b_-1 multi",
"isMultiKey" : false,
"n" : 10,
"nscannedObjects" : 20,
"nscanned" : 21,
"nscannedObjectsAllPlans" : 68,
"nscannedAllPlans" : 105,
"scanAndOrder" : true,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"a" : [
[
"foo",
"foo"
],
[
"bar",
"bar"
]
],
"b" : [
[
{
"$maxElement" : 1
},
{
"$minElement" : 1
}
]
]
},
"server" : "localhost:27017"
}
"scanAndOrder" is true which means the order in the index can't be used to
sort the return set. This means that the query will choke when given an
offset (i.e. skip 10000) and subsequently will return "too much data for
sort() with no index. add an index or specify a smaller limit". When the
query is altered to only do a single equality check, the index is used as
expected:
db.articles.find({ a: "foo" }).sort({b: -1}).limit(10).explain()
The resultset order is now the order the documents have in the index:
"scanAndOrder" : false
So it seems to have to do with how the "$in" operator behaves with index
lookups?