AWR and ASH, used properly

What each is for, the sections worth reading, and THE TRAP OF COMPARING A BAD PERIOD AGAINST NOTHING. AWR is a summary of a period; ASH is a sample of what individual sessions were doing. They answer different questions and reaching for the wrong one is most of the wasted effort here. AWR ANSWERS "WHAT DOMINATED THIS HOUR". It aggregates, so it is the right tool for a general complaint — the system was slow this morning — and the wrong tool for one report that failed at 10:42. ASH ANSWERS "WHAT WAS THIS SESSION DOING AT THAT MOMENT". It samples active sessions, so it can show a spike that an hour-long average completely conceals — and it is the tool for a specific incident at a specific time. THE SECTIONS WORTH READING IN AN AWR ARE FEW AND THE REPORT IS ENORMOUS. The top wait events, which is Lesson…

In this module: Module 11 · Performance: finding the actual problem

1. 1Measure before you tuneFree preview
2. 2"The system is slow": turning that into a question
3. 3Where time actually goes
4. 4AWR and ASH, used properly
5. 5A baseline is what makes a report meaningful
6. 6Finding the expensive SQL
7. 7Execution plans, enough to act
8. 8Statistics: the commonest real cause
9. 9Locking and blocking
10. 10When it is the application, not the database
11. 11What breaks: four performance investigations that went wrong
12. 12Lab briefing · Diagnose three slow systems