Database Error - Table Handling

[Mean]

I 've impexed my MyBB yo vBulletin.. Impex files and mybb are still in ftp.
MyBb and Vbulletin is in the same Database.

mybb is on default root
vbulletin is in /forum

Error is this >

PHP Code:

Database error in vBulletin 3.6.7:

Invalid SQL:

        SELECT
            user.username, (user.options & 512) AS invisible, user.usergroupid,
            session.userid, session.inforum, session.lastactivity,
            IF(displaygroupid=0, user.usergroupid, displaygroupid) AS displaygroupid, infractiongroupid
        FROM vb_session AS session
        LEFT JOIN vb_user AS user ON(user.userid = session.userid)
        WHERE session.lastactivity > 1182594355
        ORDER BY username ASC;

MySQL Error  : Got error 12 from table handler
Error Number : 1030
Date         : Saturday, June 23rd 2007 @ 01:40:56 PM
Script       : http://www.forumistan.net/forum/index.php?
Referrer     : http://www.forumistan.net/forum/showthread.php?p=7397#post7397
IP Address   : xxxxx
Username     : xxxxx
Classname    : vb_database 

[Lynne]

You may want to take a look at this thread: http://www.vbulletin.com/forum/showthread.php?t=118580

[Mean]

Sir I cannot understand what he said about ?

I don't really remember what the cause of this was. Does mysqld run up to 99% and jamms up? If so, try tracking the query that takes all of the resources. I recently solved that prob by checking out slow queries of mysl. In /etc/my.cnf add the
log-slow-queries=/var/log/mysql-slow.log
set-variable = long_query_time=10
log-long-format # this will log selects not using indexes