As explained in the comments for generate_orderedappend_paths, we don't
currently support parameterized MergeAppend paths, and it doesn't seem
like going to change anytime soon. Based on that, we could simplify
create_merge_append_path a bit, such as set param_info to NULL directly
rather than call get_appendrel_parampathinfo() for it. We already have
an Assert on that in create_merge_append_plan.
I understand that the change would not make any difference for
performance, it's just for clarity's sake.
Thanks
Richard
