i am trying to understand some parts of the catalyst optimizer. but i struggle with one bigger picture issue:
LogicalPlan extends TreeNode, which makes sense since the optimizations rely on tree transformations like transformUp and transformDown. but how can a LogicalPlan be a tree? isnt it really a DAG? if it is possible to create diamond-like operator dependencies, then assumptions made in tree transformations could be wrong? for example pushing a limit operator down into a child sounds safe, but if that same child is also used by another operator (so it has another parent, no longer a tree) then its not safe at all. what am i missing here? thanks! koert
