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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$active_session_history会每秒钟将数据库所有节点的Active Session采样一次，
而dba_hist_active_sess_history则会将gv$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