Post-merge fixes#2:

- A certain problem with NULLs filtering was solved in two different 
  ways. Use MWL#128's approach (it's more generic) and rollback DS-MRR's.

sql/sql_select.cc

@@ -6723,6 +6723,7 @@ make_join_select(JOIN *join,SQL_SELECT *select,COND *cond)
   DBUG_ENTER("make_join_select");
   if (select)
   {
+    add_not_null_conds(join);
     table_map used_tables;
     /*
       Step #1: Extract constant condition
@@ -7178,7 +7179,6 @@ make_join_select(JOIN *join,SQL_SELECT *select,COND *cond)
       }
 
     }
-    add_not_null_conds(join);
   }
   DBUG_RETURN(0);
 }