MDEV-7950 Item_func::type() takes 0.26% in OLTP RO

Step #3: Splitting the function check_equality() into a method in Item.
Implementing Item::check_equality() and Item_func_eq::check_equality().
Implement Item_func_eq::build_equal_items() in addition to
Item_func::build_equal_items() and moving the call for check_equality()
from Item_func::build_equal_items() to Item_func_eq::build_equal_items().

sql/item.h

@@ -1133,6 +1133,16 @@ public:
     update_used_tables();
     return this;
   }
+  /*
+    Checks whether the item is:
+    - a simple equality (field=field_item or field=constant_item), or
+    - a row equality
+    and form multiple equality predicates.
+  */
+  virtual bool check_equality(THD *thd, COND_EQUAL *cond, List<Item> *eq_list)
+  {
+    return false;
+  }
   virtual void split_sum_func(THD *thd, Item **ref_pointer_array,
                               List<Item> &fields) {}
   /* Called for items that really have to be split */

sql/item_cmpfunc.h

@@ -562,6 +562,9 @@ public:
   cond_result eq_cmp_result() const { return COND_TRUE; }
   const char *func_name() const { return "="; }
   Item *negated_item();
+  COND *build_equal_items(THD *thd, COND_EQUAL *inherited,
+                          bool link_item_fields);
+  bool check_equality(THD *thd, COND_EQUAL *cond, List<Item> *eq_list);
   /* 
     - If this equality is created from the subquery's IN-equality:
       number of the item it was created from, e.g. for

sql/sql_select.cc

@@ -12685,14 +12685,11 @@ static bool check_row_equality(THD *thd, Item *left_row, Item_row *right_row,
            or, if the procedure fails by a fatal error.
 */
 
-static bool check_equality(THD *thd, Item *item, COND_EQUAL *cond_equal,
+bool Item_func_eq::check_equality(THD *thd, COND_EQUAL *cond_equal,
                                   List<Item> *eq_list)
 {
-  if (item->type() == Item::FUNC_ITEM &&
-         ((Item_func*) item)->functype() == Item_func::EQ_FUNC)
-  {
-    Item *left_item= ((Item_func*) item)->arguments()[0];
-    Item *right_item= ((Item_func*) item)->arguments()[1];
+  Item *left_item= arguments()[0];
+  Item *right_item= arguments()[1];
 
   if (left_item->type() == Item::ROW_ITEM &&
       right_item->type() == Item::ROW_ITEM)
@@ -12702,10 +12699,7 @@ static bool check_equality(THD *thd, Item *item, COND_EQUAL *cond_equal,
                               (Item_row *) right_item,
                               cond_equal, eq_list);
   }
-    else 
-      return check_simple_equality(left_item, right_item, item, cond_equal);
-  } 
-  return FALSE;
+  return check_simple_equality(left_item, right_item, this, cond_equal);
 }
 
                           
@@ -12804,7 +12798,7 @@ COND *Item_cond_and::build_equal_items(THD *thd,
       structure here because it's restored before each
       re-execution of any prepared statement/stored procedure.
     */
-    if (check_equality(thd, item, &cond_equal, &eq_list))
+    if (item->check_equality(thd, &cond_equal, &eq_list))
       li.remove();
   }
 
@@ -12893,7 +12887,7 @@ COND *Item_cond::build_equal_items(THD *thd,
 }
 
 
-COND *Item_func::build_equal_items(THD *thd,
+COND *Item_func_eq::build_equal_items(THD *thd,
                                       COND_EQUAL *inherited,
                                       bool link_item_fields)
 {
@@ -12910,7 +12904,7 @@ COND *Item_func::build_equal_items(THD *thd,
     for WHERE a=b AND c=d AND (b=c OR d=5)
     b=c is replaced by =(a,b,c,d).  
    */
-  if (check_equality(thd, this, &cond_equal, &eq_list))
+  if (Item_func_eq::check_equality(thd, &cond_equal, &eq_list))
   {
     Item_equal *item_equal;
     int n= cond_equal.current_level.elements + eq_list.elements;
@@ -12955,6 +12949,13 @@ COND *Item_func::build_equal_items(THD *thd,
       return and_cond;
     }
   }
+  return Item_func::build_equal_items(thd, inherited, link_item_fields);
+}
+
+
+COND *Item_func::build_equal_items(THD *thd, COND_EQUAL *inherited,
+                                   bool link_item_fields)
+{
   /* 
     For each field reference in cond, not from equal item predicates,
     set a pointer to the multiple equality it belongs to (if there is any)