"ORDER BY" without large table scans and filesort

[Andreas]

Erm ... threadid is the PRIMARY index of table thread
Although it IMHO doesn't make sense to add another index to the column, i did so - no change.

Schema
[sql]
CREATE TABLE thread (
threadid int(10) unsigned NOT NULL auto_increment,
title varchar(250) NOT NULL default '',
firstpostid int(10) unsigned NOT NULL default '0',
lastpost int(10) unsigned NOT NULL default '0',
forumid smallint(5) unsigned NOT NULL default '0',
pollid int(10) unsigned NOT NULL default '0',
`open` smallint(6) NOT NULL default '0',
replycount int(10) unsigned NOT NULL default '0',
postusername varchar(100) NOT NULL default '',
postuserid int(10) unsigned NOT NULL default '0',
lastposter varchar(50) NOT NULL default '',
dateline int(10) unsigned NOT NULL default '0',
views int(10) unsigned NOT NULL default '0',
iconid smallint(5) unsigned NOT NULL default '0',
notes varchar(250) NOT NULL default '',
visible smallint(6) NOT NULL default '0',
sticky smallint(6) NOT NULL default '0',
votenum smallint(5) unsigned NOT NULL default '0',
votetotal smallint(5) unsigned NOT NULL default '0',
attach smallint(5) unsigned NOT NULL default '0',
similar varchar(55) NOT NULL default '',
PRIMARY KEY (threadid),
UNIQUE KEY threadid (threadid),
KEY postuserid (postuserid),
KEY pollid (pollid),
KEY forumid (forumid,visible,sticky,lastpost),
KEY lastpost (lastpost,forumid)
)
[/sql]