Remove not used mem_root argument from build_clone(), get_copy() and …

…get_item_copy()

TODO:
- Make get_thd_memroot() inline
  - To do this, we need to reduce dependence of include files, especially
    so that sql_class.h is not depending in item.h

sql/item.cc

@@ -78,6 +78,12 @@ inline void set_max_sum_func_level(THD *thd, SELECT_LEX *select)
                   select->nest_level - 1);
 }
 
+
+MEM_ROOT *get_thd_memroot(THD *thd)
+{
+  return thd->mem_root;
+}
+
 /*****************************************************************************
 ** Item functions
 *****************************************************************************/
@@ -2700,15 +2706,15 @@ bool Type_std_attributes::agg_item_set_converter(const DTCollation &coll,
      0 if an error occured
 */ 
 
-Item* Item_func_or_sum::build_clone(THD *thd, MEM_ROOT *mem_root)
+Item* Item_func_or_sum::build_clone(THD *thd)
 {
-  Item_func_or_sum *copy= (Item_func_or_sum *) get_copy(thd, mem_root);
+  Item_func_or_sum *copy= (Item_func_or_sum *) get_copy(thd);
   if (!copy)
     return 0;
   if (arg_count > 2)
   {
     copy->args= 
-      (Item**) alloc_root(mem_root, sizeof(Item*) * arg_count);
+      (Item**) alloc_root(thd->mem_root, sizeof(Item*) * arg_count);
     if (!copy->args)
       return 0;
   }
@@ -2718,7 +2724,7 @@ Item* Item_func_or_sum::build_clone(THD *thd, MEM_ROOT *mem_root)
 
   for (uint i= 0; i < arg_count; i++)
   {
-    Item *arg_clone= args[i]->build_clone(thd, mem_root);
+    Item *arg_clone= args[i]->build_clone(thd);
     if (!arg_clone)
       return 0;
     copy->args[i]= arg_clone;
@@ -2743,19 +2749,13 @@ Item* Item_func_or_sum::build_clone(THD *thd, MEM_ROOT *mem_root)
      0 if an error occured
 */ 
 
-Item* Item_ref::build_clone(THD *thd, MEM_ROOT *mem_root)
+Item* Item_ref::build_clone(THD *thd)
 {
-  Item_ref *copy= (Item_ref *) get_copy(thd, mem_root);
-  if (!copy)
-    return 0;
-  copy->ref= 
-      (Item**) alloc_root(mem_root, sizeof(Item*));
-  if (!copy->ref)
-      return 0;
-  Item *item_clone= (* ref)->build_clone(thd, mem_root);
-  if (!item_clone)
+  Item_ref *copy= (Item_ref *) get_copy(thd);
+  if (!copy ||
+      !(copy->ref= (Item**) alloc_root(thd->mem_root, sizeof(Item*))) ||
+      !(*copy->ref= (* ref)->build_clone(thd)))
     return 0;
-  *copy->ref= item_clone;
   return copy;
 }
 
@@ -7342,7 +7342,7 @@ Item *Item_field::derived_field_transformer_for_where(THD *thd, uchar *arg)
   st_select_lex *sel= (st_select_lex *)arg;
   Item *producing_item= find_producing_item(this, sel);
   if (producing_item)
-    return producing_item->build_clone(thd, thd->mem_root);
+    return producing_item->build_clone(thd);
   return this;
 }
 
@@ -7354,7 +7354,7 @@ Item *Item_direct_view_ref::derived_field_transformer_for_where(THD *thd,
     st_select_lex *sel= (st_select_lex *)arg;
     Item *producing_item= find_producing_item(this, sel);
     DBUG_ASSERT (producing_item != NULL);
-    return producing_item->build_clone(thd, thd->mem_root);
+    return producing_item->build_clone(thd);
   }
   return this;
 }
@@ -7400,7 +7400,7 @@ Item *Item_field::derived_grouping_field_transformer_for_where(THD *thd,
   st_select_lex *sel= (st_select_lex *)arg;
   Grouping_tmp_field *gr_field= find_matching_grouping_field(this, sel);
   if (gr_field)
-    return gr_field->producing_item->build_clone(thd, thd->mem_root);
+    return gr_field->producing_item->build_clone(thd);
   return this;
 }
 
@@ -7413,7 +7413,7 @@ Item_direct_view_ref::derived_grouping_field_transformer_for_where(THD *thd,
     return this;
   st_select_lex *sel= (st_select_lex *)arg;
   Grouping_tmp_field *gr_field= find_matching_grouping_field(this, sel);
-  return gr_field->producing_item->build_clone(thd, thd->mem_root);
+  return gr_field->producing_item->build_clone(thd);
 }
 
 void Item_field::print(String *str, enum_query_type query_type)