It looks to me like set_append_rel_pathlist builds a partial path for
an appendrel only if there exist partial paths for all members of the
appendrel. I don't understand this restriction. Why could you not
build it from a regular path for each appendrel child? You'd have
to farm out entire childrels to workers, which might be larger chunks
of work than one could wish, but it seems better than failing to
parallelize at all.
regards, tom lane