Top Sessions
The Top Sessions section lists the sessions that were waiting for the wait event that accounted for the highest percentages of sampled session activity. Use this information to identify the sessions that may be the cause of the performance problem.
The # Samples Active column shows the number of ASH samples in which the session was found waiting for that particular event. The percentage is calculated based on wall-clock time.
In Figure 8-5, the # Samples Active column shows that of the 300 times that ASH sampled database activity, the HR session (SID 123) performed a sequential read 243 times and a flashback operation 36 times. So, HR was active at least 93% of the time. The session consumed 27% of the total activity (much less than 93%) because other sessions, including the SH session, were also active.
It appears that the HR and SH sessions were running the high-load SQL statement in Figure 8-4. You should investigate this session to determine whether it is performing a legitimate operation and tune the SQL statement if possible. If tuning the SQL is not possible, and if a session is causing an unacceptable performance impact on the system, then consider terminating the session.
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