By accident I had a wrong reduce method:
map = function()

{ emit(this.ln, 1); }

reduce = function(key, vals) { var sum = 0; for (var val in vals)

{ sum += val; }

return sum; }

The rows in collection actually have only 1 entry per this.ln.
If using inline the results look like:

{ "_id" : "zzucdarlws", "value" : 1 }

If output goes to collection, it's
> db.output.find(

{"_id": "zzucdarlws"}

)

{ "_id" : "zzucdarlws", "value" : "00" }

It looks like for inline, the reduce function is never called, whereas it's called once for the collection.