SELECT left(non_default_charset_field,n) GROUP BY 1

now works more correctly. Still needs fixes.

sql/item.cc

@@ -133,6 +133,9 @@ void Item_field::set_field(Field *field_par)
   field_name=field_par->field_name;
   binary=field_par->binary();
   unsigned_flag=test(field_par->flags & UNSIGNED_FLAG);
+  /* For string fields copy character set from original field */
+  if (!field_par->binary())
+    str_value.set_charset(((Field_str*)field_par)->charset());
 }
 
 const char *Item_ident::full_name() const

sql/item_func.cc

@@ -78,6 +78,17 @@ Item_func::fix_fields(THD *thd,TABLE_LIST *tables)
 	maybe_null=1;
       if ((*arg)->binary)
 	binary=1;
+      /*
+	Change charset to arg charset if it is not equal to 
+	default_charset_info. This will work for many cases, 
+	but generally this should be done more carefull. Each string 
+	function should have it's own fix_fields() method to correctly
+	setup it's result's character set taking in account arguments.
+	For example: left(a,b) should take in account only first argument,
+	but ignore the second one.
+      */
+      if ((*arg)->str_value.charset() != default_charset_info)
+	str_value.set_charset((*arg)->str_value.charset());
       with_sum_func= with_sum_func || (*arg)->with_sum_func;
       used_tables_cache|=(*arg)->used_tables();
       const_item_cache&= (*arg)->const_item();

sql/item_strfunc.cc

@@ -759,6 +759,7 @@ String *Item_func_left::val_str(String *str)
     if (!res->alloced_length())
     {						// Don't change const str
       str_value= *res;				// Not malloced string
+      str_value.set_charset(res->charset());
       res= &str_value;
     }
     res->length((uint) length);

sql/sql_select.cc

@@ -3579,7 +3579,8 @@ Field *create_tmp_field(THD *thd, TABLE *table,Item *item, Item::Type type,
 				   item->name,table,item->binary);
       else
 	new_field= new Field_string(item->max_length,maybe_null,
-				    item->name,table,item->binary,default_charset_info);
+				    item->name,table,item->binary,
+				    item->str_value.charset());
       break;
     }
     if (copy_func)