> That's right. The varattno is set to zero for whole-row Var. And in this > case these whole-row Vars are not included in the targetlist. > > Attached is an attempt for the fix.
Wow, this is very interesting. I was surprised that this patch was necessary at all -- I mean, if wholerow refs don't work, then why do references to any other columns work? The answer is that parse_merge.c is already setting up the subplan's targetlist by expanding all vars of the source relation. I then remembered than in Simon's (or Pavan's) original coding, parse_merge.c had a hack to include a var with the source's wholerow in that targetlist, which I had later removed ...
At first I was wondering whether we need to also include vars used in each action's targetlist, just as what we did for each action's qual. Then later I realized parse_merge.c already did that. But now it looks much better to process them two in preprocess_targetlist.
I eventually realized that there's no need for parse_merge.c to expand the source rel at all, and indeed it's wasteful: we can just let preprocess_targetlist include the vars that are referenced by either quals or each action's targetlist instead. That led me to the attached patch, which is not commit-quality yet but it should show what I have in mind.
This patch looks in a good shape to me.
A minor comment is that we can use list_concat_copy(list1, list2) instead of list_concat(list_copy(list1), list2) for better efficiency.