Improve comments in the optimizer

sql/sql_select.cc

@@ -26818,6 +26818,17 @@ change_to_use_tmp_fields(THD *thd, Ref_ptr_array ref_pointer_array,
   for (uint i= 0; (item= it++); i++)
   {
     Field *field;
+    /*
+      SUM_FUNC_ITEM will be replaced by the calculated value which is
+      stored in the temporary table.
+      The first part of the following test is for items that are expressions
+      with SUM_FUNC_ITEMS, like 'sum(a)+1'. In this case we keep the original
+      item, which contain an Item_ref that points to the SUM_FUNC_ITEM that
+      will be replaced with a pointer to the calculated value.
+      The second test is for window functions. Window functions contains
+      only pointers to Item_refs, which will be adjusted to point to the
+      temporary table.
+    */
     enum Item::Type item_type= item->type();
     if ((item->with_sum_func() && item_type != Item::SUM_FUNC_ITEM) ||
        item->with_window_func())

sql/sql_window.cc

@@ -423,6 +423,16 @@ ORDER *st_select_lex::find_common_window_func_partition_fields(THD *thd)
 #define CMP_GT_C       1    // Greater than and compatible
 #define CMP_GT         2    // Greater then
 
+
+/*
+  This function is used for sorting ORDER/PARTITION BY clauses of window
+  functions and so must implement an order relation on ORDER BY clauses"
+
+  It is called by a sorting function.
+  The function return's CMP_EQ (=0) if the values are identical.
+  If not equal, it returns a stable value < or > than 0.
+*/
+
 static
 int compare_order_elements(ORDER *ord1, int weight1,
                            ORDER *ord2, int weight2)