This plan type is widely used in reporting queries, so will hit the mainline of BI applications and many Mat View creations. This will allow SELECT count(*) FROM foo to go faster also.
We'd also need to add some infrastructure to merge aggregate states together for this to work properly. This means that could also work for avg() and stddev etc. For max() and min() the merge functions would likely just be the same as the transition functions.
It might make sense to make a new planner operator which can be responsible for pulling from each of the individual parallel Agg nodes and then aggregating over the results.
A couple of things that might be worth considering are whether we want to enforce using parallel aggregation or let planner decide if it wants to do a parallel aggregate or go with a single plan. For eg, the average estimated size of groups might be one thing that planner may consider while deciding between a parallel and a single execution plan.
I dont see merging states as an easy problem, and should perhaps be tackled apart from this thread.
Also, do we want to allow parallelism only with GroupAggs?