ashis an active sessionhistory.
it only captures data about activesessions.
ifa session becomes idle after taking a lock then details will not be recordedand so are not reported in any reports produced
ash是对active状态的session的采样(每秒采样).也就是说:
在oracle10g中，引入了awr和ash采样机制，有一个视图gv$active_session_history会每秒钟将所有节点的active session采样一次，
而dba_hist_active_sess_history则会将gv$active_session_history里的数据每10秒采样一次并持久化保存
https://blogs.oracle.com/database4cn/dbahistactivesesshistory