Please add this to relativity.install

    case 'pgsql':
      db_query("CREATE TABLE {relativity} (
          nid INTEGER DEFAULT 0 NOT NULL, 
          parent_nid INTEGER DEFAULT 0 NOT NULL, 
          PRIMARY KEY(nid, parent_nid))");

      db_query("CREATE TABLE {relativity_query} (
          nid INTEGER DEFAULT 0 NOT NULL, 
          target_nid INTEGER, 
          follow_parents INTEGER, 
          follow_children INTEGER, 
          recursion_depth INTEGER, 
          unique_types INTEGER, 
          max_results INTEGER, 
          options TEXT, 
          search_algorithm VARCHAR(20) DEFAULT 'dfs', 
          search_types TEXT, 
          end_pts TEXT, 
          avoid_pts TEXT, 
          PRIMARY KEY(nid)");