Robert, Ashutosh, any comments on this? I'm unfamiliar with the partitionwise join code.
As the comment says it has to do with the equivalence classes being used during merge append. EC's are used to create pathkeys used for sorting. Creating a sort node which has column on the nullable side of an OUTER join will fail if it doesn't find corresponding equivalence class. You may not notice this if both the partitions being joined are pruned for some reason. Amit's idea to make partition-wise join code do this may work, but will add a similar overhead esp. in N-way partition-wise join once those equivalence classes are added.
I looked at the patch. The problem there is that for a given relation, we will add child ec member multiple times, as many times as the number of joins it participates in. We need to avoid that to keep ec_member list length in check.
