Locking and blocking

One session waiting on another, FINDING THE HEAD OF THE CHAIN, and THE CONVERSATION BEFORE YOU KILL ANYTHING. Blocking is the performance problem that looks least like one: the machine is idle, every resource metric is healthy, and users cannot work. Lesson 3's third category, and it is diagnosed differently from everything else in this module. THE CHAIN IS THE THING TO FIND, BECAUSE WHAT USERS REPORT IS THE TAIL OF IT. Twenty sessions are stuck; nineteen of them are waiting on each other. ONE session at the head is blocking everything and is not waiting for anything at all — it is idle, holding a lock, doing nothing. SO KILLING THE SESSION SOMEBODY COMPLAINED ABOUT ACHIEVES NOTHING. It is a victim. The chain reforms behind the same head. Walk to the head before doing anything, which is a query the database answers directly. AND THE HEAD IS USUALLY A…

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