Prevent needless rexecuting empty subselects

sql/item_subselect.cc

@@ -36,7 +36,7 @@ SUBSELECT TODO:
 #include "sql_select.h"
 
 Item_subselect::Item_subselect(THD *thd, st_select_lex *select_lex):
-  executed(0), optimized(0), error(0)
+  assigned(0), executed(0), optimized(0), error(0)
 {
   DBUG_ENTER("Item_subselect::Item_subselect");
   DBUG_PRINT("subs", ("select_lex 0x%xl", (long) select_lex));
@@ -141,7 +141,7 @@ int Item_subselect::exec()
       return 1;
     }
     assign_null();
-    executed= 0;
+    executed= assigned= 0;
   }
   if (!executed)
   {
@@ -149,6 +149,7 @@ int Item_subselect::exec()
     join->thd->lex.select= select_lex;
     join->exec();
     join->thd->lex.select= save_select;
+    executed= 1;
     return join->error;
   }
   return 0;

sql/item_subselect.h

@@ -31,6 +31,7 @@ class Item_subselect :public Item
 protected:
   longlong int_value;
   double real_value;
+  my_bool assigned; /* value already assigned to subselect */
   my_bool executed; /* simple subselect is executed */
   my_bool optimized; /* simple subselect is optimized */
   my_bool error; /* error in query */
@@ -59,6 +60,7 @@ public:
     max_length= item->max_length;
     decimals= item->decimals;
     res_type= item->res_type;
+    assigned= item->assigned;
     executed= item->executed;
     select_lex= item->select_lex;
     join= item->join;

sql/sql_class.cc

@@ -785,7 +785,7 @@ select_subselect::select_subselect(Item_subselect *item)
 bool select_subselect::send_data(List<Item> &items)
 {
   DBUG_ENTER("select_subselect::send_data");
-  if (item->executed){
+  if (item->assigned){
     my_printf_error(ER_SUBSELECT_NO_1_ROW, ER(ER_SUBSELECT_NO_1_ROW), MYF(0));
     DBUG_RETURN(1);
   }
@@ -812,6 +812,6 @@ bool select_subselect::send_data(List<Item> &items)
     item->int_value= val_item->val_int();
     item->res_type= val_item->result_type();
   }
-  item->executed= 1;
+  item->assigned= 1;
   DBUG_RETURN(0);
 }