Hi Huiyong,
It implies that due to outdated statistics, execution plan is leading to high row fetches, increasing CPU and IO costs.
Further investigation needed to identify the following
1) Update the database statistics
2) Check whether correct indexes are picked during the SQL fetch.
For more information on analysis refer link
http://sapbasisdurgaprasad.blogspot.in/2011/07/identifying-expensive-sqls-using-st04.html
Hope this helps.
Regards,
Deepak Kori