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CHAPTER 7 CONCURRENCY AND MULTI-VERSIONING
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6 / Trigger created. ops$tkyte%ORA11GR2> update t set x = x+1; fired 1 row updated. and go into that second session again and run the update, we observe it gets blocked (of course). After committing the blocking session, we ll see the following: ops$tkyte%ORA11GR2> update t set x = x+1 where y > 0; fired 1 row updated. The trigger fired just once this time, not twice. Thus, the :NEW and :OLD column values, when referenced in the trigger, are also used by Oracle to do the restart checking. When we referenced :NEW.X and :OLD.X in the trigger, X s consistent read and current read values were compared and found to be different. A restart ensued. When we removed the reference to that column from the trigger, there was no restart. So the rule is that the set of columns used in the WHERE clause to find the rows plus the columns referenced in the row triggers will be compared. The consistent read version of the row will be compared to the current read version of the row; if any of them are different, the modification will restart.
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FOR EACH ROW trigger is more efficient than using a BEFORE FOR EACH ROW. The AFTER trigger won t have the same effect we ve already retrieved the block in current mode by then.
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Note You can use this bit of information to further understand why using an AFTER
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Which leads us to the Why do we care question.
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Why Is a Restart Important to Us
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The first thing that pops out should be Our trigger fired twice! We had a one-row table with a BEFORE FOR EACH ROW trigger on it. We updated one row, yet the trigger fired two times. Think of the potential implications of this. If you have a trigger that does anything nontransactional, this could be a fairly serious issue. For example, consider a trigger that sends an update where the body of the e-mail is This is what the data used to look like. It has been modified to look like this now. If you sent the e-mail directly from the trigger, using UTL_SMTP in Oracle9i or UTL_MAIL in Oracle 10g and above, then the user would receive two e-mails, with one of them reporting an update that never actually happened. Anything you do in a trigger that is nontransactional will be impacted by a restart. Consider the following implications: Consider a trigger that maintains some PL/SQL global variables, such as for the number of rows processed. When a statement that restarts rolls back, the modifications to PL/SQL variables won t roll back.
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CHAPTER 7 CONCURRENCY AND MULTI-VERSIONING
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Virtually any function that starts with UTL_ (UTL_FILE, UTL_HTTP, UTL_SMTP, and so on) should be considered susceptible to a statement restart. When the statement restarts, UTL_FILE won t un-write to the file it was writing to. Any trigger that is part of an autonomous transaction must be suspect. When the statement restarts and rolls back, the autonomous transaction can t be rolled back.
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All of these consequences must be handled with care in the belief that they may be fired more than once per row or be fired for a row that won t be updated by the statement after all. The second reason you should care about potential restarts is performance related. We have been using a single-row example, but what happens if you start a large batch update and it is restarted after processing the first 100,000 records It will roll back the 100,000 row changes, restart in SELECT FOR UPDATE mode, and do the 100,000 row changes again after that. You might notice, after putting in that simple audit trail trigger (the one that reads the :NEW and :OLD values), that performance is much worse than you can explain, even though nothing else has changed except the new triggers. It could be that you are restarting queries you never used in the past. Or the addition of a tiny program that updates just a single row here and there makes a batch process that used to run in an hour suddenly run in many hours due to restarts that never used to take place. This is not a new feature of Oracle it has been in the database since version 4.0, when read consistency was introduced. I myself was not totally aware of how it worked until the summer of 2003 and, after I discovered what it implied, I was able to answer a lot of How could that have happened questions from my own past. It has made me swear off using autonomous transactions in triggers almost entirely, and it has made me rethink the way some of my applications have been implemented. For example, I ll never send e-mail from a trigger directly; rather, I ll always use DBMS_JOB or something similar to send the e-mail after my transaction commits. This makes the sending of the e-mail transactional; that is, if the statement that caused the trigger to fire and send the e-mail is restarted, the rollback it performs will roll back the DBMS_JOB request. Most everything nontransactional that I did in triggers was modified to be done in a job after the fact, making it all transactionally consistent.
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Summary
In this chapter, we covered a lot of material that, at times, might not have been obvious. However, it is vital that you understand these issues. For example, if you were not aware of the statement-level restart, you might not be able to figure out how a certain set of circumstances could have taken place. That is, you would not be able to explain some of the daily empirical observations you make. In fact, if you were not aware of the restarts, you might wrongly suspect the actual fault to be due to the circumstances or end user error. It would be one of those unreproducible issues, as it takes many things happening in a specific order to observe. We took a look at the meaning of the isolation levels set out in the SQL standard and at how Oracle implements them; at times, we contrasted Oracle s implementation with that of other databases. We saw that in other implementations (i.e., ones that employ read locks to provide consistent data), there is a huge trade-off between concurrency and consistency. To get highly concurrent access to data, you would have to decrease your need for consistent answers. To get consistent, correct answers, you would need to live with decreased concurrency. In Oracle that is not the case, due to its multiversioning feature. Table 7-9 sums up what you might expect in a database that employs read locking versus Oracle s multi-versioning approach.
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