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CHAPTER 7 CONCURRENCY AND MULTI-VERSIONING
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Note Oracle attempts to do this purely at the row level, but you may receive an ORA-08177 error even when the
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row you are interested in modifying has not been modified. The ORA-08177 error may happen due to some other row(s) being modified on the block that contains your row.
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Oracle takes an optimistic approach to serialization it gambles on the fact that the data your transaction wants to update won t be updated by any other transaction. This is typically the way it happens, and usually the gamble pays off, especially in quick-transaction, OLTP-type systems. If no one else updates your data during your transaction, this isolation level, which will generally decrease concurrency in other systems, will provide the same degree of concurrency as it would without SERIALIZABLE transactions. The downside to this is that you may get the ORA-08177 error if the gamble doesn t pay off. If you think about it, however, it s worth the risk. If you re using SERIALIZABLE transactions, you shouldn t expect to update the same information as other transactions. If you do, you should use the SELECT ... FOR UPDATE as described in 1 Developing Successful Oracle Applications , and this will serialize the access. So, using an isolation level of SERIALIZABLE will be achievable and effective if you: Have a high probability of no one else modifying the same data. Need transaction-level read consistency. Will be doing short transactions (to help make the first bullet point a reality).
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Oracle finds this method scalable enough to run all of their TPC-Cs (an industry standard OLTP benchmark; see http://www.tpc.org for details). In many other implementations, you will find this being achieved with shared read locks and their corresponding deadlocks, and blocking. In Oracle, we do not get any blocking, but we will get the ORA-08177 error if other sessions change the data we want to change as well. However, we will not get the error as frequently as we will get deadlocks and blocks in the other systems. But there is always a but you must take care to understand these different isolation levels and their implications. Remember, with isolation set to SERIALIZABLE, you will not see any changes made in the database after the start of your transaction, until you commit. Applications that attempt to enforce their own data integrity constraints, such as the resource scheduler described in 1 Developing Successful Oracle Applications , must take extra care in this regard. If you recall, the problem in 1 was that we could not enforce our integrity constraint in a multiuser system since we could not see changes made by other uncommitted sessions. Using SERIALIZABLE, we would still not see the uncommitted changes, but we would also not see the committed changes made after our transaction began! As a final point, be aware that SERIALIZABLE does not mean that all transactions executed by users will behave as if they were executed one right after another in a serial fashion. It does not imply that there is some serial ordering of the transactions that will result in the same outcome. The phenomena previously described by the SQL standard do not make this happen. This last point is a frequently misunderstood concept, and a small demonstration will clear it up. The following table represents two sessions performing work over time. The database tables A and B start out empty and are created as follows: ops$tkyte@ORA11GR2> create table a ( x int ); Table created. ops$tkyte@ORA11GR2> create table b ( x int ); Table created.
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CHAPTER 7 CONCURRENCY AND MULTI-VERSIONING
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Now we have the series of events shown in Table 7-7. Table 7-7. SERIALIZABLE Transaction Example
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Time
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Session 1 Executes
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Alter session set isolation_level=serializable; --
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Session 2 Executes
Alter session set isolation_level=serializable; -Insert into b select count(*) from a; -Commit;
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