Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 7 ■ CONCURRENCY AND MULTI-VERSIONING 241 in Chapter 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$tkyte@ORA10G> create table a ( x int ); Table created. ops$tkyte@ORA10G> create table b ( x int ); Table created. Now we have the series of events shown in Table 7-7. Table 7-7. SERIALIZABLE Transaction Example Time Session 1 Executes Session 2 Executes T1 T2 Alter session set isolation_level= serializable; Alter session set isolation_level= serializable; T3 Insert into a select count(*) from b; T4 Insert into b select count(*) from a; T5 Commit; T6 Commit; Now, when all is said and done, tables A and B will each have a row with the value 0 in it. If there was some “serial” ordering of the transactions, we could not possibly have both tables containing the value 0 in them. If session 1 executed before session 2, then table B would have a row with the value 1 in it. If session 2 executed before session 1, then table A would have a row with the value 1 in it. As executed here, however, both tables will have rows with a value of 0. They just executed as if they were the only transaction in the database at that point in time. No matter how many times session 1 queries table B, the count will be the count that was committed in the database at time T1. Likewise, no matter how many times session 2 queries table A, the count will be the same as it was at time T2. READ ONLY READ ONLY transactions are very similar to SERIALIZABLE transactions, the only difference being that they do not allow modifications, so they are not susceptible to the ORA-08177 error. READ ONLY transactions are intended to support reporting needs, where the contents of the
242 CHAPTER 7 ■ CONCURRENCY AND MULTI-VERSIONING report need to be consistent with respect to a single point in time. In other systems, you would use REPEATABLE READ and suffer the associated affects of the shared read lock. In Oracle, you will use the READ ONLY transaction. In this mode, the output you produce in a report that uses 50 SELECT statements to gather the data will be consistent with respect to a single point in time—the time the transaction began. You will be able to do this without locking a single piece of data anywhere. This aim is achieved by using the same multi-versioning as used for individual statements. The data is reconstructed as needed from the rollback segments and presented to you as it existed when the report began. READ ONLY transactions are not trouble-free, however. Whereas you might see an ORA-08177 error in a SERIALIZABLE transaction, you expect to see an ORA-1555: snapshot too old error with READ ONLY transactions. This will happen on a system where other people are actively modifying the information you are reading. The changes (undo) made to this information are recorded in the rollback segments. But rollback segments are used in a circular fashion in much the same manner as redo logs. The longer the report takes to run, the better the chance that some undo you need to reconstruct your data won’t be there anymore. The rollback segment will have wrapped around, and the portion of it you need we be reused by some other transaction. At this point, you will receive the ORA-1555 error and have to start over again. The only solution to this sticky issue is to have rollback segments that are sized correctly for your system. Time and time again, I see people trying to save a few megabytes of disk space by having the smallest possible rollback segments (“Why ‘waste’ space on something I don’t really need?” is the thought). The problem is that the rollback segments are a key component of the way the database works, and unless they are sized correctly, you will hit this error. In 16 years of using Oracle 6, 7, and 8, I can say I have never hit an ORA-1555 error outside of a testing or development system. In such a case, you know you have not sized the rollback segments correctly and you fix it. We will revisit this issue in Chapter 9. Implications of Multi-Version Read Consistency So far, we’ve seen how multi-versioning provides us with non-blocking reads, and I have stressed that this is a good thing: consistent (correct) answers with a high degree of concurrency. What could be wrong with that? Well, unless you understand that it exists and what it implies, then you are probably doing some of your transactions incorrectly. Recall from Chapter 1 the scheduling resources example whereby we had to employ some manual locking techniques (via SELECT FOR UPDATE to serialize modifications to the SCHEDULES table by resource). But can it affect us in other ways? The answer to that is definitely yes. We’ll go into the specifics in the sections that follow. A Common Data Warehousing Technique That Fails A common data warehousing technique I’ve seen people employ goes like this: 1. They use a trigger to maintain a LAST_UPDATED column in the source table, much like the method described in the last chapter, in the “Optimistic Locking” section. 2. To initially populate a data warehouse table, they remember what time it is right now by selecting out SYSDATE on the source system. For example, suppose it is exactly 9:00 am right now.
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Expert Oracle Database Architecture
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Contents Foreword . . . . . . . . .
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■CONTENTS v Block Buffer Cache .
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■CONTENTS vii ■CHAPTER 9 Redo a
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Foreword “THINK.” In 1914, Thom
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■FOREWORD xiii Tom is an aficiona
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About the Technical Reviewers ■JO
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Introduction The inspiration for th
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■INTRODUCTION xix • Exposure to
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■INTRODUCTION xxi Chapter 3: File
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■INTRODUCTION xxiii Next up are t
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Setting Up Your Environment In this
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