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

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CHAPTER 6 ■ LOCKING AND LATCHING 191 7 not null, 8 constraint dept_pk primary key(deptno) 9 ) 10 / Table created. Then we INSERT a copy of the DEPT data into this table: ops$tkyte@ORA10G> insert into dept( deptno, dname, loc ) 2 select deptno, dname, loc 3 from scott.dept; 4 rows created. ops$tkyte@ORA10G> commit; Commit complete. That code re-creates the DEPT table, but with an additional LAST_MOD column that uses the TIMESTAMP WITH TIME ZONE datatype (available in Oracle9i and above). We have defined this column to be NOT NULL so that it must be populated, and its default value is the current system time. This TIMESTAMP datatype has the highest precision in Oracle, typically going down to the microsecond (millionth of a second). For an application that involves user think time, this level of precision on the TIMESTAMP is more than sufficient, as it is highly unlikely that the process of the database retrieving a row and a human looking at it, modifying it, and issuing the update back to the database could take place within a fraction of a second. The odds of two people reading and modifying the same row in the same fraction of a second are very small indeed. Next, we need way of maintaining this value. We have two choices: either the application can maintain the LAST_MOD column by setting its value to SYSTIMESTAMP when it updates a record or a trigger/stored procedure can maintain it. Having the application maintain LAST_MOD is definitely more performant than a trigger-based approach, since a trigger will add additional processing on part of Oracle to the modification. However, this does mean that you are relying on all of the applications to maintain LAST_MOD consistently in all of the places that they modify this table. So, if each application is responsible for maintaining this field, it needs to consistently verify that the LAST_MOD column was not changed and set the LAST_MOD column to the current SYSTIMESTAMP. For example, if an application queries the row where DEPTNO=10 ops$tkyte@ORA10G> variable deptno number ops$tkyte@ORA10G> variable dname varchar2(14) ops$tkyte@ORA10G> variable loc varchar2(13) ops$tkyte@ORA10G> variable last_mod varchar2(50) ops$tkyte@ORA10G> begin 2 :deptno := 10; 3 select dname, loc, last_mod 4 into :dname,:loc,:last_mod 5 from dept 6 where deptno = :deptno;
192 CHAPTER 6 ■ LOCKING AND LATCHING 7 end; 8 / PL/SQL procedure successfully completed. which we can see is currently ops$tkyte@ORA10G> select :deptno dno, :dname dname, :loc loc, :last_mod lm 2 from dual; DNO DNAME LOC LM ---------- ---------- -------- ----------------------------------- 10 ACCOUNTING NEW YORK 25-APR-05 10.54.00.493380 AM -04:00 it would use this next update statement to modify the information. The last line does the very important check to make sure the timestamp has not changed and uses the built-in function TO_TIMESTAMP_TZ (TZ is short for TimeZone) to convert the string we saved in from the select back into the proper datatype. Additionally, line 3 of the update updates the LAST_MOD column to be the current time if the row is found to be updated: ops$tkyte@ORA10G> update dept 2 set dname = initcap(:dname), 3 last_mod = systimestamp 4 where deptno = :deptno 5 and last_mod = to_timestamp_tz(:last_mod); 1 row updated. As you can see, one row was updated—the row of interest. We updated the row by primary key (DEPTNO) and verified that the LAST_MOD column had not been modified by any other session between the time we read it first and the time we did the update. If we were to try to update that same record again, using the same logic, but without retrieving the new LAST_MOD value, we would observe the following: ops$tkyte@ORA10G> update dept 2 set dname = upper(:dname), 3 last_mod = systimestamp 4 where deptno = :deptno 5 and last_mod = to_timestamp_tz(:last_mod); 0 rows updated. Notice how 0 rows updated is reported this time because the predicate on LAST_MOD was not satisfied. While DEPTNO 10 still exists, the value at the moment we wish to update no longer matches the timestamp value at the moment we queried the row. So, the application knows, based on the fact that no rows were modified, that the data has been changed in the database— and it must now figure out what it wants to do about that. You would not rely on each application to maintain this field for a number of reasons. For one, it adds code to an application, and it is code that must be repeated and correctly implemented anywhere this table is modified. In a large application, that could be in many places. Furthermore, every application developed in the future must conform to these rules. There are many chances to “miss” a spot in the application code and not have this field properly used.
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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 3 ■ FILES 89 A Brief Revi
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CHAPTER 3 ■ FILES 91 Redundant Ar
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CHAPTER 3 ■ FILES 95 dictionary t
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CHAPTER 3 ■ FILES 113 tkyte@ORA10
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CHAPTER 4 ■ ■ ■ Memory Struct
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CHAPTER 4 ■ MEMORY STRUCTURES 117
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284 CHAPTER 9 ■ REDO AND UNDO cri
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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 11 ■ ■ ■ Indexes Inde
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CHAPTER 13 ■ ■ ■ Partitioning
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Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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