Encapsulate use of List<Cached_item> in a Group_bound_tracker.

sql/item_windowfunc.cc

@@ -164,12 +164,7 @@ void Item_window_func::split_sum_func(THD *thd, Ref_ptr_array ref_pointer_array,
 
 void Item_window_func::setup_partition_border_check(THD *thd)
 {
-  for (ORDER *curr= window_spec->partition_list->first; curr; curr=curr->next)
-  {
-    //curr->item_ptr->fix_fields(thd, curr->item);
-    Cached_item *tmp= new_Cached_item(thd, curr->item[0], TRUE);  
-    partition_fields.push_back(tmp);
-  }
+  partition_tracker.init(thd, window_spec->partition_list);
   window_func()->setup_window_func(thd, window_spec);
 }
 
@@ -208,16 +203,14 @@ bool Item_sum_rank::add()
   return false; 
 }
 
-int Item_window_func::check_partition_bound()
+bool Item_window_func::check_if_partition_changed()
 {
-  return test_if_group_changed(partition_fields);
+  return partition_tracker.check_if_next_group();
 }
 
 void Item_window_func::advance_window()
 {
-  int changed= check_partition_bound();
-
-  if (changed > -1)
+  if (check_if_partition_changed())
   {
     /* Next partition */
     window_func()->clear();

sql/item_windowfunc.h

@@ -485,7 +485,7 @@ public:
     advance_window() uses this to tell when we've left one partition and
     entered another
   */
-  List<Cached_item> partition_fields;
+  Group_bound_tracker partition_tracker;
 public:
   Item_window_func(THD *thd, Item_sum *win_func, LEX_STRING *win_name)
     : Item_func_or_sum(thd, (Item *) win_func),
@@ -537,7 +537,7 @@ public:
   void setup_partition_border_check(THD *thd);
 
   void advance_window();
-  int check_partition_bound();
+  bool check_if_partition_changed();
 
   enum_field_types field_type() const
   { 

sql/sql_window.cc

@@ -1281,8 +1281,8 @@ bool compute_window_func_with_frames(Item_window_func *item_win,
     {
       break; /* End of file */
     }
-    bool partition_changed= (item_win->check_partition_bound() > -1)? true:
-                                                                     false;
+    bool partition_changed= item_win->check_if_partition_changed();
+
     tbl->file->position(tbl->record[0]);
     memcpy(rowid_buf, tbl->file->ref, tbl->file->ref_length);
 
@@ -1382,8 +1382,7 @@ bool compute_two_pass_window_functions(Item_window_func *item_win,
       done = true;
     }
 
-    bool partition_changed= (done || item_win->check_partition_bound() > -1) ?
-                              true : false;
+    bool partition_changed= done || item_win->check_if_partition_changed();
     // The first time we always have a partition changed. Ignore it.
     if (first_row)
     {