CSC#4385: slow sorting for UTF8 large table:

my_strnxfrm_utf8 now requires 2 bytes per character
in filesort key, instead of 3 bytes per character.
Shorter filesort keys make sorting faster.

strings/ctype-simple.c

@@ -21,6 +21,15 @@
 
 #include "stdarg.h"
 
+/*
+  Returns the number of bytes required for strnxfrm().
+*/
+uint my_strnxfrmlen_simple(CHARSET_INFO *cs, uint len)
+{
+  return len * (cs->strxfrm_multiply ? cs->strxfrm_multiply : 1);
+}
+
+
 /*
   Converts a string into its sort key.
   
@@ -1365,6 +1374,7 @@ MY_COLLATION_HANDLER my_collation_8bit_simple_ci_handler =
     my_strnncoll_simple,
     my_strnncollsp_simple,
     my_strnxfrm_simple,
+    my_strnxfrmlen_simple,
     my_like_range_simple,
     my_wildcmp_8bit,
     my_strcasecmp_8bit,