Added comments

sql/item_windowfunc.cc

@@ -5,6 +5,12 @@ Item_window_func::fix_fields(THD *thd, Item **ref)
 {
   DBUG_ASSERT(fixed == 0);
   
+  /*
+    TODO: why the last parameter is 'ref' in this call? What if window_func
+    decides to substitute itself for something else and does *ref=.... ? 
+    This will substitute *this (an Item_window_func object) with Item_sum
+    object. Is this the intent?
+  */
   if (window_func->fix_fields(thd, ref))
     return TRUE;
 

sql/item_windowfunc.h

@@ -157,6 +157,24 @@ class Item_window_func : public Item_result_field
 
   enum_field_types field_type() const { return window_func->field_type(); }
   
+  /* 
+    TODO: Window functions are very special functions, so val_() methods have
+    special meaning for them:
+
+    - Phase#1: we run the join and put its result into temporary table. For 
+      window functions, we write NULL (or some other) values as placeholders.
+      
+    - Phase#2: executor does the scan in {PARTITION, ORDER BY} order of this 
+      window function. It calls appropriate methods to inform the window
+      function about rows entering/leaving the window. 
+      It calls window_func->val_int() so that current window function value 
+      can be saved and stored in the temp.table.
+
+    - Phase#3: the temporaty table is read and passed to query output. (Do 
+      I understand correctly that Item_window_func::val_XXX won't be called 
+      at all in this phase? Need to check)
+
+  */
   double val_real() {  return window_func->val_real(); }
 
   longlong val_int() { return window_func->val_int(); }