ASH (Active Session History) is a series of snapshots of the v$sessions and v$session_wait views over time - used to track session activity and simplify performance tuning. Snapshot are taken every second and stored in memory (v$active_session_history) for about 30 minutes. After that the data is flushed to the AWR (dba_hist_active_sess_history table).
ASH was first introduced in Oracle 10g.
Licensing ASH may not be used unless Enterprise Manager Diagnostic Pack is licensed.
Sample queries Top CPU consumers (last 5 minutes):
SELECT session_id, count(*) FROM v$active_session_history WHERE session_state= 'ON CPU' AND SAMPLE_TIME > sysdate - (5/(24*60)) GROUP BY session_id ORDER BY count(*) desc; Top waiting sessions (last 5 minutes): SELECT session_id, count(*) FROM v$active_session_history WHERE session_state= 'WAITING' AND SAMPLE_TIME > sysdate - (5/(24*60)) GROUP BY session_id ORDER BY count(*) desc;