this is absolutely broken.
i am trying count(1) with hive.map.aggr = true - and there is no map side aggregation happening (even though the explain has a map-side group by operator):
Alias -> Map Operator Tree:
Group By Operator
Reduce Output Operator
Map-reduce partition columns:
it seems that the groupbyDesc doe not have a 'keys' field specified (in other map side aggregates - i can see the keys specified).
At any rate - the mapper emits one output row for each input row in this case. This is completely broken ..