mysql如何优化这条更新语句？

Question

    UPDATE yw_tmp_usr a SET a.hth=(SELECT b.hth FROM yhdang b WHERE a.business_key=b.dh )
    WHERE EXISTS (SELECT b.hth FROM yhdang b WHERE a.business_key=b.dh );

上面这条语句在Oracle中执行效率比mysql中高得多，在mysql中却总是没反应，甚至报错：1205。我修改了linux上的my.cnf文件，让innodb_lock_wait_timeout = 1200，但仍无结果，请问各位大神，8.5万的数据，这样更新大概需要多长时间，如何优化呢？？

Answer 1

UPDATE yw_tmp_usr a, yhdang b SET a.hth = b.hth WHERE a.business_key=b.dh

Answer 2

UPDATE yw_tmp_usr SET hth = T.hth

FROM (

SELECT hth, dh FROM yhdang  

) T

WHERE yw_tmp_usr.business_key = T.dh