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

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CHAPTER 8 ■ TRANSACTIONS 257 • ROLLBACK TO : This statement is used with the SAVEPOINT command. You may roll back your transaction to that marked point without rolling back any of the work that preceded it. So, you could issue two UPDATE statements, followed by a SAVEPOINT and then two DELETE statements. If an error or some sort of exceptional condition occurs during execution of the DELETE statements, and you catch that exception and issue the ROLLBACK TO SAVEPOINT command, the transaction will roll back to the named SAVEPOINT, undoing any work performed by the DELETEs, but leaving the work performed by the UPDATE statements intact. • SET TRANSACTION: This statement allows you to set various transaction attributes, such as the transaction’s isolation level and whether it is read-only or read-write. You can also use this statement to instruct the transaction to use a specific undo segment when using manual undo management, but this is not recommended. We’ll discuss manual and automatic undo management in more detail in Chapter 9. That’s it—there are no more transaction control statements. The most frequently used control statements are COMMIT and ROLLBACK. The SAVEPOINT statement has a somewhat special purpose. Internally, Oracle uses it frequently, and you may find some use for it in your application as well. Atomicity After that brief overview of the transaction control statements, we’re ready to see what’s meant by statement, procedure, and transaction atomicity. Statement-Level Atomicity Consider the following statement: Insert into t values ( 1 ); It seems fairly clear that if it were to fail due to a constraint violation, the row would not be inserted. However, consider the following example, where an INSERT or DELETE on table T fires a trigger that adjusts the CNT column in table T2 appropriately: ops$tkyte@ORA10G> create table t2 ( cnt int ); Table created. ops$tkyte@ORA10G> insert into t2 values ( 0 ); 1 row created. ops$tkyte@ORA10G> commit; Commit complete. ops$tkyte@ORA10G> create table t ( x int check ( x>0 ) ); Table created. ops$tkyte@ORA10G> create trigger t_trigger 2 before insert or delete on t for each row
258 CHAPTER 8 ■ TRANSACTIONS 3 begin 4 if ( inserting ) then 5 update t2 set cnt = cnt +1; 6 else 7 update t2 set cnt = cnt -1; 8 end if; 9 dbms_output.put_line( 'I fired and updated ' || 10 sql%rowcount || ' rows' ); 11 end; 12 / Trigger created. In this situation, it is less clear what should happen. If the error occurs after the trigger has fired, should the effects of the trigger be there or not? That is, if the trigger fired and updated T2, but the row was not inserted into T, what should the outcome be? Clearly the answer is that we would not like the CNT column in T2 to be incremented if a row is not actually inserted into T. Fortunately in Oracle, the original statement from the client—INSERT INTO T, in this case—either entirely succeeds or entirely fails. This statement is atomic. We can confirm this, as follows: ops$tkyte@ORA10G> set serveroutput on ops$tkyte@ORA10G> insert into t values (1); I fired and updated 1 rows 1 row created. ops$tkyte@ORA10G> insert into t values(-1); I fired and updated 1 rows insert into t values(-1) * ERROR at line 1: ORA-02290: check constraint (OPS$TKYTE.SYS_C009597) violated ops$tkyte@ORA10G> select * from t2; CNT ---------- 1 ■Note When using SQL*Plus from Oracle9i Release 2 and before, in order to see that the trigger fired, you will need to add a line of code, exec null, after the second insert. This is because SQL*Plus does not retrieve and display the DBMS_OUTPUT information after a failed DML statement in those releases. In Oracle 10g it does.
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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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CHAPTER 13 ■ ■ ■ Partitioning
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