cleanup: only do work when needed

sql/item.cc

@@ -2384,10 +2384,6 @@ left_is_superset(const DTCollation *left, const DTCollation *right)
 
 bool DTCollation::aggregate(const DTCollation &dt, uint flags)
 {
-
-  THD *thd = current_thd;
-  myf utf8_flag= thd ? thd->get_utf8_flag() :
-                 global_system_variables.old_behavior & OLD_MODE_UTF8_IS_UTF8MB3;
   if (!my_charset_same(collation, dt.collation))
   {
     /* 
@@ -2475,6 +2471,9 @@ bool DTCollation::aggregate(const DTCollation &dt, uint flags)
         set(dt);
         return 0;
       }
+      THD *thd = current_thd;
+      myf utf8_flag= thd ? thd->get_utf8_flag()
+        : global_system_variables.old_behavior & OLD_MODE_UTF8_IS_UTF8MB3;
       CHARSET_INFO *bin= get_charset_by_csname(collation->cs_name.str,
                                                MY_CS_BINSORT,MYF(utf8_flag));
       set(bin, DERIVATION_NONE);