select tm from (
select (opti_history.oh_date-opti_threads.ot_time_opened) as tm, (@R:=@R+1) r from opti_history, opti_threads where opti_history.oh_type = 1 AND opti_history.oh_action IN (4,7) AND opti_history.oh_oid = opti_threads.ot_id AND opti_history.oh_date > 1337904000 AND ot_pid = 4 order by tm asc)
t where r=((select count(*)+1 from opti_history, opti_threads where  opti_history.oh_type = 1 AND opti_history.oh_action IN (4,7) AND opti_history.oh_oid = opti_threads.ot_id AND opti_history.oh_date > 1337904000 AND ot_pid = 4) DIV 2)