how to avoid filesort on union all query - mysql

Question

Please i have this query which uses filesort if combined(union all), but if executed separately works fine without file sort. Here i dont want to execute it separately but want to implement the union all option. Please someone should help.
(SELECT STRAIGHT_JOIN *,
topic_post_time,
topic_title,
topic_id AS tid,
p.userid AS profile_id,
Concat(first_name, ' ', last_name) AS poster_name,
Concat(first_name, '.', last_name) AS profile_name,
forum_id,
topic_last_post_time,
sch_name_abbrev,
picture_small_url,
profile_pix_upload_path,
profile_pix_upload_path,
LEFT(post_text, 100) AS post_text
FROM _forum_topics FORCE INDEX(topic_poster)
INNER JOIN _profile p
ON ( p.userid = _forum_topics.topic_poster )
INNER JOIN _users
ON p.userid = _users.userid
INNER JOIN _class
ON _users.classid = _class.classid
INNER JOIN _unit
ON _class.unitid = _unit.unitid
INNER JOIN _department
ON _unit.deptid = _department.deptid
INNER JOIN _faculty
ON _department.facid = _faculty.facid
INNER JOIN _university
ON ( _faculty.schid = _university.schid )
WHERE _forum_topics.sub_forum_id IN( 133, 134, 135, 136,
137, 138 )
ORDER BY topic_last_post_time DESC
LIMIT 0, 20)
UNION ALL
(SELECT STRAIGHT_JOIN *,
topic_post_time,
topic_title,
topic_id AS tid,
p.userid AS profile_id,
Concat(first_name, ' ', last_name) AS poster_name,
Concat(first_name, '.', last_name) AS profile_name,
forum_id,
topic_last_post_time,
sch_name_abbrev,
picture_small_url,
profile_pix_upload_path,
profile_pix_upload_path,
LEFT(post_text, 100) AS post_text
FROM _sch_forum_topics s FORCE INDEX(topic_poster)
INNER JOIN _profile p
ON ( p.userid = s.topic_poster )
INNER JOIN _users
ON p.userid = _users.userid
INNER JOIN _class
ON _users.classid = _class.classid
INNER JOIN _unit
ON _class.unitid = _unit.unitid
INNER JOIN _department
ON _unit.deptid = _department.deptid
INNER JOIN _faculty
ON _department.facid = _faculty.facid
INNER JOIN _university
ON _faculty.schid = _university.schid
ORDER BY topic_last_post_time DESC
LIMIT 0, 20)
LIMIT
0, 20

Answer 1

It seems to me that the only difference is the WHERE clause. Just use an OR.
Also, ORDER BY is not accomplished in this context, not because MySQL is buggy, but because it does not have a logical meaning.
There is also an old MySQL bug which makes all UNIONs use temporary tables:
http://bugs.mysql.com/bug.php?id=50674
and ordering a temporary table generated by a query means filesort

Answer 2

#Federico
"There is also an old MySQL bug which makes all UNIONs use temporary tables: http://bugs.mysql.com/bug.php?id=50674 and ordering a temporary table generated by a query means filesort"
I've made an anwser because an comment doenst allow code formatting
C++ Source code off (file sql/sql_union.cc) MySQL 5.5.32 union looks like it always creates an temporary table..
See the code comments and function create_tmp_table
/*
Create a temporary table to store the result of select_union.
SYNOPSIS
select_union::create_result_table()
thd thread handle
column_types a list of items used to define columns of the
temporary table
is_union_distinct if set, the temporary table will eliminate
duplicates on insert
options create options
DESCRIPTION
Create a temporary table that is used to store the result of a UNION,
derived table, or a materialized cursor.
RETURN VALUE
0 The table has been created successfully.
1 create_tmp_table failed.
*/
bool
select_union::create_result_table(THD *thd_arg, List<Item> *column_types,
bool is_union_distinct, ulonglong options,
const char *alias)
{
DBUG_ASSERT(table == 0);
tmp_table_param.init();
tmp_table_param.field_count= column_types->elements;
if (! (table= create_tmp_table(thd_arg, &tmp_table_param, *column_types,
(ORDER*) 0, is_union_distinct, 1,
options, HA_POS_ERROR, alias)))