Move table record writing outside of loop

We can set values in the record buffer first and only perform one table
write call at the end. No need to write to file every time one column is
updated.

Also, remove unused method from Table_read_cursor.

sql/sql_window.cc

@@ -630,18 +630,6 @@ class Table_read_cursor : public Rowid_seq_cursor
     return table->file->ha_rnd_pos(record, curr_rowid);
   }
 
-  bool fetch_prev_row()
-  {
-    uchar *p;
-    if ((p= get_prev_rowid()))
-    {
-      int rc= table->file->ha_rnd_pos(record, p);
-      if (!rc)
-        return true; // restored ok
-    }
-    return false; // didn't restore
-  }
-
 private:
   /* The table that is acccesed by this cursor. */
   TABLE *table;
@@ -2136,14 +2124,11 @@ bool save_window_function_values(List<Item_window_func>& window_functions,
   tbl->file->ha_rnd_pos(tbl->record[0], rowid_buf);
   store_record(tbl, record[1]);
   while (Item_window_func *item_win= iter++)
-  {
-    int err;
     item_win->save_in_field(item_win->result_field, true);
-    // TODO check if this can be placed outside the loop.
-    err= tbl->file->ha_update_row(tbl->record[1], tbl->record[0]);
-    if (err && err != HA_ERR_RECORD_IS_THE_SAME)
-      return true;
-  }
+
+  int err= tbl->file->ha_update_row(tbl->record[1], tbl->record[0]);
+  if (err && err != HA_ERR_RECORD_IS_THE_SAME)
+    return true;
 
   return false;
 }