[置顶] undo丢失且存在未提交事务的恢复

详见原文博客地址  http://www.killdb.com/2012/01/31/undo%e4%b8%a2%e5%a4%b1%e4%b8%94%e5%ad%98%e5%9c%a8%e6%9c%aa%e6%8f%90%e4%ba%a4%e4%ba%8b%e5%8a%a1%e7%9a%84%e6%81%a2%e5%a4%8d.html