finally pushing what I could not due to BK problems

Docs/manual.texi

@@ -50815,6 +50815,10 @@ each individual 4.0.x release.
 @appendixsubsec Changes in release 4.0.5
 @itemize
 @item
+Removed a condition that temp table with index on column that can be NULL has
+to be MyISAM. This was OK for 3.23, but not needed in 4.*. This resulted in 
+slowdown in many queries since 4.0.2
+@item
 Small code improvement in multi-table updates
 @item
 Fixed a newly introduced bug that caused @code{ORDER BY ... LIMIT #}

sql/sql_select.cc

@@ -3708,7 +3708,7 @@ create_tmp_table(THD *thd,TMP_TABLE_PARAM *param,List<Item> &fields,
   *blob_field= 0;				// End marker
 
   /* If result table is small; use a heap */
-  if (blob_count || using_unique_constraint || group_null_items ||
+  if (blob_count || using_unique_constraint ||
       (select_options & (OPTION_BIG_TABLES | SELECT_SMALL_RESULT)) ==
       OPTION_BIG_TABLES)
   {