There's currently a dassert that fails if this scenario arises. I think that parent/child dependencies (or more generally 'ancestral' dependencies like "a" and "a.b.c") should probably disqualify a COLUMN_SCAN since we will need to go to the row store probably to re-assemble the parent. This may not be the case in all cases, but should be a fairly good heuristic for the time being. For example I could imagine querying for {a.nested: {$exists: false}} and somehow knowing that means that in all matching results a will be a scalar value.

I think fixing it to work in this case would not be too bad, but it's probably not something that would be beneficial most of the time.