If I run a sort query (with limit) on a collection in ascending order, everything operates properly:

> db.signatures.find().sort(

{numbb:1}

).limit(10)

{ "_id" : ObjectId("4b980bcb5506ca18cc00000d"), "numbb" : 2, "symbol" : "gen_raw_REG", "type" : "ssig", "signature" : BinData type: 2 len: 67 }

.....

But I actually want descending order:
> db.signatures.find().sort(

{numbb:-1}

).limit(10)
error:

{ "$err" : "too much data for sort() with no index" }

Curiously, a smaller limit works properly:
> db.signatures.find().sort(

{numbb:-1}

).limit(3)

{ "_id" : ObjectId("4b980ed75506ca18cc02d6fb"), "_types" : [ "Signature" ], "symbol" : "_ZN8LargeNet13doBuildInsideEv", "numbb" : 6323, "_cls" : "Signature", "signature" : BinData type: 2 len: 2364674, "type" : "ssig" }

....

For reference, .limit(4) is apparently too large.

If there's a good reason for this, so be it. I'm guessing, however, that this is due to the query optimizer doing quite different things depending on sort order.