I'm confused about this. The comment on the function seems to indicate that there is absolutely no shuffle or network IO but it also states that it assigns an even number of parent partitions to each final partition group. I'm having trouble seeing how this can be guaranteed without some data passing around nodes.
For instance, lets saying I have 5 machines and 10 partitions but the way the partitions are layed out is machines 1, 2, and 3 each have 3 partitions while machine 4 only has 1 partition and machine 5 has none. Am I to assume that coalesce(4, false) will the 3 partitions on nodes 1, 2, and 3 each to 1 partition while node 4 will just remain 1 partition? Thanks.
