Major Description
This flag forces our execution engine to perform aggregations on mapper nodes rather than on reducer.
First, we already have enough information in runtime to decide whether we can optimize or not. Second, if query is complex enough, users would have hard time trying to figure whether query would return correct result or not. We should never ever return invalid results.
Let's make our engine smarter, so that it is able to determine whether such grouping is safe or not and then deprecate that flag.