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Lois: Hello and welcome back to the Oracle University Podcast. I'm Lois Houston, Director of Innovation Programs with Oracle University, and with me is Nikita Abraham, Team Lead of Editorial Services.
Nikita: Hi everyone! Last week, we looked at two Oracle Database 23ai new features related to Data Manipulation Language, or DML. One was Unrestricted Parallel DMLs and the other was Unrestricted Direct Loads. Do check out that episode if you missed it.
Lois: Today, we have Senior Principal Database & MySQL Instructor, Bill Millar, with us. He's been on several times this season taking us through all the different 23ai new features. In this episode, we're going to ask him about the Automatic Transaction Rollback feature. Hi Bill! What is automatic transaction rollback and why is it an important feature for database administrators?
Bill: We can now have transactions that are blocking other transactions, depending on some settings, to automatically roll back. It does require some parameters to be set. Rows basically get locked in a single row. Each row is locked based off of what type of activity is being performed on that row, such as inserts, updates, deletes, merge, select for updates.
Nikita: And how were things before this feature?
Bill: Traditionally, the database administrator had to research and manually terminate blocking transactions, or there are some things that resource manager might have been able to do.
Lois: This seems like such a game-changer for DBAs, Bill. So, how does it work?
Bill: So there are some parameters that control the automatic rollback. One is the transaction priority. We're going to set that priority for a transaction either to medium, high, or low. We have the high priority wait target and a medium priority wait target that we can set.
The high wait target will terminate if a medium transaction is blocking that high target based off of the values that we set, the medium transaction can be terminated. A medium transaction will terminate a low priority. So if a transaction designated as low exceeds the blocking time that we set for the medium priority wait time, then it'll be terminated. Whereas, the high priority will terminate both medium and low transactions.
We have the rollback mode. We're either going to roll back or we're going to track, depending on what we're trying to do.
Nikita: So, if I decide that I want to use automatic transaction rollback… if I decide to implement it…I'll need to set those parameters, right?
Bill: So we can set those at a session level. We also have some system level wait targets. What are the wait times for the medium, high transactions? How long they are going to wait for those lower transactions?
And then we also have the rollback mode. Are we actually going to roll back or are we just going to track for right now? We have to determine what is going to be the wait times for those transactions that we want to wait before those lower transactions, priority transactions are rolled back?
At that session level, we're going to set the session. High is the default. So if we want transactions to run at a lower, we have to set those. So we can set the medium or low because that's going to determine how they're rolled back.
So, what is that rollback order? Again the low, we'll roll back any low that's blocking mediums. High, we'll roll back any mediums or lows that are blocking. So you do need to have the understanding of that application, and how critical are the different transactions, because if you start rolling back transactions, what? It does-- If you roll back the transactions, it does generate a little research, a little bit more work on why did that happen.
Lois: Yeah… you don't want to set it without really understanding what you're doing. Ok, so, what else do I need to know?
Bill: So we do have the system level wait targets again. How long is the high priority transaction going to wait for a lower transaction before it rolls it back? How long that medium priority is going to wait?
We use the ALTER SYSTEM SET command. It does have a range of values from one second to 2,147,483,647 seconds. That's like 68 years. Might not want to wait 68 years for a transaction to be rolled back.
We can set it at the PDB level. Each pluggable can have a different value. And it can have a different value in the different RAC instances. We have those system level wait targets that we want to set. Automatic rollback. In order for it to function, all the parameters have to be set properly.
What is that transaction priority? We saw the medium, high, low. What is the wait target? How long is the medium is going to wait? How long is the low is going to wait? We set that in seconds. The order of those transactions determine how they are terminated.
Lois: Earlier on, you mentioned rollback mode. Can you tell us a little more about it?
Bill: So with that automatic rollback mode, there's only two valid values. It is considered advanced parameter. We can either set it in rollback, which is the default, or we can put it in track mode. Track mode gives us the ability to try it out. I guess you can say.
It will say, hey, if I would have been running, if I would have been used, I would have terminated this transaction. It'll show me the number of times it would have happened for high priority, the number of times it would happen for a medium priority. It is modifiable in the PDB, but however, the track mode must be the same in each instance.
So that rollback mode, again, that is the default value for that. So statistics are going to be available. So how many high priority rollbacks occurred? How many medium rollbacks occurred?
In that track mode, I have to set that value. I do have to have the time set for how long is it going to wait for those, so the high and medium. And those priorities has to be set in the session.
So statistics are available for the high and the medium in the track mode. Not only when we're actually rolling back, but also tracking. Again, this gives us the ability, by having it in the track mode, gives us the ability to do a little testing with it first.
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Nikita: Welcome back! Bill, when it comes to monitoring, how do you keep track of these rollbacks?
Bill: For monitoring our rollback transactions, the data dictionary information is available to assist with monitoring our transaction priority. So from the V$TRANSACTIONS, there are columns available allowing us to do that.
Based off that transaction priority shows what is the wait target for that. And then also each of the priority of those transactions. We can view this information, it will be populated to the alert log. So we can see that session ID, what was session ID of that? What was the transaction ID? What was the priority? What was the system identifier for that?
It tells you-- even tells you the parameter and tells you what that wait time was set at. If it was a medium transaction that was terminated, it shows, OK, it was a medium. So we can view the alert log. And we can look for these terminations. Gives an idea of what's being done.
Nikita: And finally, what are the key advantages of using automatic transaction rollback?
Bill: It eliminates a very manual process. It can be very time-consuming for the DBA to go out there and try to find what's the blocking session.
Yep, I'll go ahead and do an ALTER SYSTEM. I'll kill that session trying to track it down, finding the views to look at it to say, OK, Yeah, this is the blocking one. I want to go ahead and take care of it. Resource manager doesn't really fully address blocking transactions.
Some things that can do for that. We have the maximum estimate execution time. So that's the number in CPU seconds allowed for that call. It's terminated. It doesn't matter whether it's blocking another session or not in that case or even another transaction. It just says, OK, you exceeded this time. I'm going to terminate you.
Then we also have the max idle time again. That's maximum session idle time. All right. You haven't been doing anything to session, we're going to terminate you. And then we have the MAX_IDLE_BLOCKER. That's the time duration of an idle session can block another session. Again, it's going to check OK, is the session actually idle? But these don't really address the issue of, hey, I have a higher priority transaction waiting for a lower transaction that's blocking it.
Lois: Thank you, Bill, for that breakdown. This feature is such a time saver.
Nikita: Yeah, and such good way to reduce the manual workload for DBAs. Thanks Bill!
Lois: To learn more about what we discussed today and view some of the demonstrations of this feature, visit mylearn.oracle.com and search for the Oracle Database 23ai New Features for Administrators course. Join us next week for a discussion on some more Oracle Database 23ai new features. Until then, this is Lois Houston…
Nikita: And Nikita Abraham signing off!
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