In order to understand what DataFusion's planner is doing, it would be helpful to have an "EXPLAIN PLAN" feature. All other database systems I have worked with have such a feature (e.g. see MySql).
I have found EXPLAIN helpful for both users but also very much developers.
The feature would look something like
Which would then print a string with the logical plan. One way to implement this would simply be to use the debugging printout of the logical and physical plans