How Much effect to add for update clause

[ligang]

1. select * from t1 where ...
2. select * from t1 where ... for update nowait

I can not see any performance difference between these 2 sql .
But I think there should be some difference because the explain plan shows me the addition optimize step for 'for update '

[JMac]

the performance overhead (if any) is immaterial/academic. Its what you are trying to do that's important. If you are just selecting and not updating then use #1. If you intend to update the row(s) selected then use # 2 (For Update), otherwise you cannot guarantee that the rows selected have not been updated in the time between you issuing the SELECT and the UPDATE.