Add a method to disable the automatic adding of values from cursors

Currently cursors automatically add values to the sum functions they
manage. There are use cases when we just want to figure out the frame
boundaries, without actually adding/removing values from them.

sql/sql_window.cc

@@ -748,6 +748,8 @@ class Partition_read_cursor : public Table_read_cursor
 class Frame_cursor : public Sql_alloc
 {
 public:
+  Frame_cursor() : sum_functions(), perform_no_action(false) {}
+
   virtual void init(READ_RECORD *info) {};
 
   bool add_sum_func(Item_sum* item)
@@ -783,18 +785,35 @@ class Frame_cursor : public Sql_alloc
 
   virtual ~Frame_cursor() {}
 
+  /*
+     Regular frame cursors add or remove values from the sum functions they
+     manage. By calling this method, they will only perform the required
+     movement within the table, but no adding/removing will happen.
+  */
+  void set_no_action()
+  {
+    perform_no_action= true;
+  }
+
 protected:
   inline void add_value_to_items()
   {
+    if (perform_no_action)
+      return;
+
     List_iterator_fast<Item_sum> it(sum_functions);
     Item_sum *item_sum;
     while ((item_sum= it++))
     {
       item_sum->add();
     }
   }
+
   inline void remove_value_from_items()
   {
+    if (perform_no_action)
+      return;
+
     List_iterator_fast<Item_sum> it(sum_functions);
     Item_sum *item_sum;
     while ((item_sum= it++))
@@ -805,6 +824,9 @@ class Frame_cursor : public Sql_alloc
 
   /* Sum functions that this cursor handles. */
   List<Item_sum> sum_functions;
+
+private:
+  bool perform_no_action;
 };
 
 /*