Subquery optimizations: backport

- Fix valgrind failure: do initialize Item::is_expensive_cache.

sql/item.cc

@@ -373,8 +373,8 @@ int Item::save_str_value_in_field(Field *field, String *result)
 
 
 Item::Item():
-  rsize(0), name(0), orig_name(0), name_length(0), fixed(0),
-  is_autogenerated_name(TRUE),
+  is_expensive_cache(-1), rsize(0), name(0), orig_name(0), name_length(0),
+  fixed(0), is_autogenerated_name(TRUE),
   collation(&my_charset_bin, DERIVATION_COERCIBLE)
 {
   marker= 0;
@@ -410,6 +410,7 @@ Item::Item():
   tables.
 */
 Item::Item(THD *thd, Item *item):
+  is_expensive_cache(-1),
   rsize(0),
   str_value(item->str_value),
   name(item->name),

sql/item.h

@@ -513,6 +513,9 @@ class Item {
 
   enum traverse_order { POSTFIX, PREFIX };
   
+  /* Cache of the result of is_expensive(). */
+  int8 is_expensive_cache;
+  
   /* Reuse size, only used by SP local variable assignment, otherwize 0 */
   uint rsize;
 
@@ -878,9 +881,6 @@ class Item {
   static CHARSET_INFO *default_charset();
   virtual CHARSET_INFO *compare_collation() { return NULL; }
 
-  /* Cache of the result of is_expensive(). */
-  int8 is_expensive_cache;
-  
   virtual bool walk(Item_processor processor, bool walk_subquery, uchar *arg)
   {
     return (this->*processor)(arg);