Oracle —— before row trigger OR after row trigger

• before trigger可以修改new value
• before trigger可以提前中止，而不需要回滚statement
• []: AFTER row triggers are slightly more efficient than BEFORE row triggers. With BEFORE row triggers, affected data blocks must be read (logical read, not physical read) once for the trigger and then again for the triggering statement[共需要两次load].

    

  Alternatively, with AFTER row triggers, the data blocks must be read only once for both the triggering statement and the trigger[只需一次load].

  另外，《Oracle编程艺术》中7.4写一致性的重启动一节指出：重启动造成before trigger执行多次，也使得after row trigger性能占优。