A small cleanup for MDEV-16309 Split ::create_tmp_field() into virtual methods in Item

These two methods:
- Item_result_field::create_tmp_field_ex()
- Item_func_user_var::create_tmp_field_ex()
had duplicate code, except that they used a different type handler.
Adding a protected method Item_result_field::create_tmp_field_ex_from_handler()
with a "const Type_handler*" parameter, and reusing it from the
two mentioned methods.

sql/item.h

@@ -3169,6 +3169,11 @@ class st_select_lex;
 
 class Item_result_field :public Item_fixed_hybrid /* Item with result field */
 {
+protected:
+  Field *create_tmp_field_ex_from_handler(MEM_ROOT *root, TABLE *table,
+                                          Tmp_field_src *src,
+                                          const Tmp_field_param *param,
+                                          const Type_handler *h);
 public:
   Field *result_field;				/* Save result here */
   Item_result_field(THD *thd): Item_fixed_hybrid(thd), result_field(0) {}
@@ -3179,7 +3184,12 @@ class Item_result_field :public Item_fixed_hybrid /* Item with result field */
   ~Item_result_field() {}			/* Required with gcc 2.95 */
   Field *get_tmp_table_field() { return result_field; }
   Field *create_tmp_field_ex(MEM_ROOT *root, TABLE *table, Tmp_field_src *src,
-                             const Tmp_field_param *param);
+                             const Tmp_field_param *param)
+  {
+    DBUG_ASSERT(fixed);
+    const Type_handler *h= type_handler()->type_handler_for_tmp_table(this);
+    return create_tmp_field_ex_from_handler(root, table, src, param, h);
+  }
   void get_tmp_field_src(Tmp_field_src *src, const Tmp_field_param *param);
   /*
     This implementation of used_tables() used by Item_avg_field and

sql/item_func.h

@@ -2732,7 +2732,12 @@ class Item_func_user_var :public Item_hybrid_func
     :Item_hybrid_func(thd, item),
     m_var_entry(item->m_var_entry), name(item->name) { }
   Field *create_tmp_field_ex(MEM_ROOT *root, TABLE *table, Tmp_field_src *src,
-                             const Tmp_field_param *param);
+                             const Tmp_field_param *param)
+  {
+    DBUG_ASSERT(fixed);
+    return create_tmp_field_ex_from_handler(root, table, src, param,
+                                            type_handler());
+  }
   Field *create_field_for_create_select(MEM_ROOT *root, TABLE *table)
   { return create_table_field_from_handler(root, table); }
   bool check_vcol_func_processor(void *arg);

sql/sql_select.cc

@@ -17740,9 +17740,13 @@ void Item_result_field::get_tmp_field_src(Tmp_field_src *src,
 }
 
 
-Field *Item_result_field::create_tmp_field_ex(MEM_ROOT *root, TABLE *table,
+Field *
+Item_result_field::create_tmp_field_ex_from_handler(
+                                          MEM_ROOT *root,
+                                          TABLE *table,
                                           Tmp_field_src *src,
-                                              const Tmp_field_param *param)
+                                          const Tmp_field_param *param,
+                                          const Type_handler *h)
 {
   /*
     Possible Item types:
@@ -17750,26 +17754,14 @@ Field *Item_result_field::create_tmp_field_ex(MEM_ROOT *root, TABLE *table,
     - Item_func
     - Item_subselect
   */
+  DBUG_ASSERT(fixed);
   DBUG_ASSERT(is_result_field());
   DBUG_ASSERT(type() != NULL_ITEM);
   get_tmp_field_src(src, param);
   Field *result;
-  if ((result= tmp_table_field_from_field_type(root, table)) &&
-      param->modify_item())
-    result_field= result;
-  return result;
-}
-
-
-Field *Item_func_user_var::create_tmp_field_ex(MEM_ROOT *root, TABLE *table,
-                                               Tmp_field_src *src,
-                                               const Tmp_field_param *param)
-{
-  DBUG_ASSERT(is_result_field());
-  DBUG_ASSERT(type() != NULL_ITEM);
-  get_tmp_field_src(src, param);
-  Field *result;
-  if ((result= create_table_field_from_handler(root, table)) &&
+  if ((result= h->make_and_init_table_field(root, &name,
+                                            Record_addr(maybe_null),
+                                            *this, table)) &&
       param->modify_item())
     result_field= result;
   return result;