Describe the bug

a fragment is as bellow:

If the numnodes of the fragment corresponding to 【exchangelode 0】 is 1,
the numnodes of the farament corresponding to 【exchangelode 1】 is also 1,
and the numnode of the fragment corresponding to 【exchangelode 4】 is 4,
the parallelism of this fragment is max (1,1,4) = 4.

However, we plan only one aggnode in the distributed execution planning phase (this is because the numnode of both leftchildsegment and regithchild segment is 1, we only need one AGG Node).
Since the concurrency of fragment is 4（In Coordinator phase), we need two layers of aggnode（in distribute planner phase）, but the node of fragment is already unchangeable。

the query will produce wrong result