text: -- 2017-09-24
  alter table tree_relationship drop column child_table;
  alter table tree_relationship drop column parent_table;
  
  -- rm overbearing constraint
  alter table tree_relationship drop constraint tree_relationship_parent_tables_check;
  alter table tree_relationship drop constraint tree_relationship_child_tables_check;
  alter table tree_relationship alter parent_tables drop not null;
  alter table tree_relationship alter child_tables drop not null;
  