Implemented a counter within Item_sum_sum

The counter keeps track of the number of items added to the sum
function. It is increased when we add a value to the sum function and
decreased when it is removed.

sql/item_sum.cc

@@ -1243,7 +1243,8 @@ Field *Item_sum_hybrid::create_tmp_field(bool group, TABLE *table,
 Item_sum_sum::Item_sum_sum(THD *thd, Item_sum_sum *item) 
   :Item_sum_num(thd, item),
    Type_handler_hybrid_field_type(item),
-   curr_dec_buff(item->curr_dec_buff)
+   curr_dec_buff(item->curr_dec_buff),
+   count(item->count)
 {
   /* TODO: check if the following assignments are really needed */
   if (Item_sum_sum::result_type() == DECIMAL_RESULT)
@@ -1265,6 +1266,7 @@ void Item_sum_sum::clear()
 {
   DBUG_ENTER("Item_sum_sum::clear");
   null_value=1;
+  count= 0;
   if (Item_sum_sum::result_type() == DECIMAL_RESULT)
   {
     curr_dec_buff= 0;
@@ -1325,6 +1327,7 @@ bool Item_sum_sum::add()
 void Item_sum_sum::add_helper(bool perform_removal)
 {
   DBUG_ENTER("Item_sum_sum::add_helper");
+
   if (Item_sum_sum::result_type() == DECIMAL_RESULT)
   {
     my_decimal value;
@@ -1332,13 +1335,20 @@ void Item_sum_sum::add_helper(bool perform_removal)
     if (!aggr->arg_is_null(true))
     {
       if (perform_removal)
+      {
+        DBUG_ASSERT(count > 0);
         my_decimal_sub(E_DEC_FATAL_ERROR, dec_buffs + (curr_dec_buff ^ 1),
                        dec_buffs + curr_dec_buff, val);
+        count--;
+      }
       else
+      {
+        count++;
         my_decimal_add(E_DEC_FATAL_ERROR, dec_buffs + (curr_dec_buff ^ 1),
                        val, dec_buffs + curr_dec_buff);
+      }
       curr_dec_buff^= 1;
-      null_value= 0;
+      null_value= (count > 0) ? 0 : 1;
     }
   }
   else
@@ -1348,7 +1358,17 @@ void Item_sum_sum::add_helper(bool perform_removal)
     else
       sum+= aggr->arg_val_real();
     if (!aggr->arg_is_null(true))
-      null_value= 0;
+    {
+      if (perform_removal)
+      {
+        DBUG_ASSERT(count > 0);
+        count--;
+      }
+      else
+        count++;
+
+      null_value= (count > 0) ? 0 : 1;
+    }
   }
   DBUG_VOID_RETURN;
 }

sql/item_sum.h

@@ -777,6 +777,7 @@ public:
 
 private:
   void add_helper(bool perform_removal);
+  ulonglong count;
 };
 
 

sql/sql_window.cc

@@ -1261,6 +1261,13 @@ bool compute_window_func_with_frames(Item_window_func *item_win,
     */
     tbl->file->ha_rnd_pos(tbl->record[0], rowid_buf);
     store_record(tbl,record[1]);
+    // TODO-cvicentiu
+    // Save in field does not make use of the null value for the sum function.
+    // If we do however set the null value to be the same as the one that
+    // the current sum function has, via say
+    // item_win->null_value = sum_func->null_value; we get an assertion failure
+    // during the sending of data within Item::send, in the case of DECIMAL_RESULT.
+    // How do we force saving of NULL values in the table?
     item_win->save_in_field(item_win->result_field, true);
     err= tbl->file->ha_update_row(tbl->record[1], tbl->record[0]);
     if (err && err != HA_ERR_RECORD_IS_THE_SAME)