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

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CHAPTER 8 ■ TRANSACTIONS 261 X ---------- 1 ops$tkyte@ORA10G> select * from t2; CNT ---------- 1 Here, we ran a block of code that ignored any and all errors, and the difference in outcome is huge. Whereas the first call to P effected no changes, here the first INSERT succeeds and the CNT column in T2 is incremented accordingly. ■Note I consider virtually all code that contains a WHEN OTHERS exception handler that does not also include a RAISE to re-raise the exception to be a bug. It silently ignores the error and it changes the transaction semantics. Catching WHEN OTHERS and translating the exception into an old-fashioned return code changes the way the database is supposed to behave. Oracle considered the “statement” to be the block that the client submitted. This statement succeeded by catching and ignoring the error itself, so the If error then rollback... didn’t come into effect and Oracle did not roll back to the SAVEPOINT after execution. Hence, the partial work performed by P was preserved. The reason that this partial work was preserved in the first place is that we have statement-level atomicity within P: each statement in P is atomic. P becomes the client of Oracle when it submits its two INSERT statements. Each INSERT either entirely succeeds or fails. This is evidenced by the fact that we can see that the trigger on T fired twice and updated T2 twice, yet the count in T2 reflects only one UPDATE. The second INSERT executed in P had an implicit SAVEPOINT wrapped around it. The difference between the two blocks of code is subtle and something you must consider in your applications. Adding an exception handler to a block of PL/SQL code can radically change its behavior. A different way to code this—one that restores the statement-level atomicity to the entire PL/SQL block—is as follows: ops$tkyte@ORA10G> begin 2 savepoint sp; 3 p; 4 exception 5 when others then 6 rollback to sp; 7 end; 8 / I fired and updated 1 rows I fired and updated 1 rows
262 CHAPTER 8 ■ TRANSACTIONS PL/SQL procedure successfully completed. ops$tkyte@ORA10G> select * from t; no rows selected ops$tkyte@ORA10G> select * from t2; CNT ---------- 0 ■Caution The preceding code represents an exceedingly bad practice. You should neither catch a WHEN OTHERS in general nor explicitly code what Oracle already provides as far as transaction semantics is concerned. Here, by mimicking the work Oracle normally does for us with the SAVEPOINT, we are able to restore the original behavior while still catching and “ignoring” the error. I provide this example for illustration only—this would be an exceeding bad coding practice. Transaction-Level Atomicity Lastly, there is the concept of transaction-level atomicity. The entire goal of a transaction, a set of SQL statements executed together as a unit of work, is to take the database from one consistent state to another consistent state. To accomplish this goal, transactions are atomic as well—the entire set of successful work performed by a transaction is either entirely committed and made permanent or rolled back and undone. Just like a statement, the transaction is an atomic unit of work. Upon receipt of “success” from the database after committing a transaction, you know that all of the work performed by the transaction has been made persistent. Integrity Constraints and Transactions It is interesting to note exactly when integrity constraints are checked. By default, integrity constraints are checked after the entire SQL statement has been processed. There are also deferrable constraints that permit the validation of integrity constraints to be postponed until either the application requests they be validated by issuing a SET CONSTRAINTS ALL IMMEDIATE command or upon issuing COMMIT. IMMEDIATE Constraints For the first part of this discussion, we will assume that constraints are in IMMEDIATE mode, which is the norm. In this case, the integrity constraints are checked immediately after the entire SQL statement has been processed. Note I used the term “SQL statement” and not just
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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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■CONTENTS ix ■CHAPTER 12 Dataty
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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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CHAPTER 1 ■ DEVELOPING SUCCESSFUL
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CHAPTER 4 ■ MEMORY STRUCTURES 117
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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 13 ■ ■ ■ Partitioning
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