Re. the "ugly hack" comments in adjust_inherited_tlist(), I'm confused: expand_targetlist() runs *after*, not before, so how could it have affected the result?
If I understand correctly, planner must have called expand_targetlist() once for the parent relation's descriptor and added any dropped columns from the parent relation. So we may not find mapped attributes for those dropped columns in the parent. I haven't actually tested this case though.
I wonder if it makes sense to actually avoid expanding on-conflict-set targetlist in case the target is a partition table and deal with it during execution, like we are doing now.
I'm obviously confused about what expand_targetlist call this comment is talking about. Anyway I wanted to make it use resjunk entries instead, but that broke some other case that I didn't have time to research yesterday. I'll get back to this soon, but in the meantime, here's what I have.
Aren't we are adding back Vars referring to the parent relation, after having converted the existing ones to use partition relation's varno? May be that works because missing attributes are already added during planning and expand_targetlist() here only adds dropped columns, which are just NULL constants.