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

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CHAPTER 9 ■ REDO AND UNDO 319 So, when LGWR is writing group 1, ARCH is reading group 2 and writing to the archive disks. When LGWR is writing group 2, ARCH is reading group 1 and writing to the archive disks. In this fashion, LGWR and ARCH each have their own dedicated devices and will not be contending with anyone, not even each other. Figure 9-6. Redo log flow Online redo log files are the one set of Oracle files that benefit the most from the use of RAW disks. If there is one type of file you might consider for RAW, log files would be it. There is much back and forth discussion on the pros and cons of using RAW versus cooked file systems. As this is not a book on DBA/SA tasks, I won’t get into them. I’ll just mention that if you are going to use RAW devices anywhere, online redo log files would be the best candidates. You never back up online redo log files, so the fact that they are on RAW partitions versus a cooked file system won’t impact any backup scripts you might have. ARCH can always turn the RAW logs into cooked file system files (you cannot use a RAW device to archive to), hence the “mystique” of RAW devices is very much minimized in this case. Temporary Tables and Redo/Undo Temporary tables are still considered a relatively new feature of Oracle, having been introduced only in Oracle8i version 8.1.5. As such, there is some confusion surrounding them, in particular in the area of logging. In Chapter 10, we will cover how and why you might use temporary tables. In this section, we will explore only the question “How do temporary tables work with respect to logging of changes?” Temporary tables generate no redo for their blocks. Therefore, an operation on a temporary table is not “recoverable.” When you modify a block in a temporary table, no record of this change will be made in the redo log files. However, temporary tables do generate undo, and the undo is logged. Hence, temporary tables will generate some redo. At first glance, it doesn’t seem to make total sense: why would they need to generate undo? This is because you can roll
320 CHAPTER 9 ■ REDO AND UNDO back to a SAVEPOINT within a transaction. You might erase the last 50 INSERTs into a temporary table, leaving the first 50. Temporary tables can have constraints and everything else a normal table can have. They might fail a statement on the five-hundredth row of a 500-row INSERT, necessitating a rollback of that statement. Since temporary tables behave in general just like “normal” tables, temporary tables must generate undo. Since undo data must be logged, temporary tables will generate some redo log for the undo they generate. This is not nearly as ominous as it seems. The primary SQL statements used against temporary tables are INSERTs and SELECTs. Fortunately, INSERTs generate very little undo (you need to restore the block to “nothing,” and it doesn’t take very much room to store “nothing”), and SELECTs generate no undo. Hence, if you use temporary tables for INSERTs and SELECTs exclusively, this section means nothing to you. It is only if you UPDATE or DELETE that you might be concerned about this. I set up a small test to demonstrate the amount of redo generated while working with temporary tables, an indication therefore of the amount of undo generated for temporary tables, since only the undo is logged for them. To demonstrate this, I will take identically configured “permanent” and “temporary” tables, and then perform the same operations on each, measuring the amount of redo generated each time. The tables I used were simply as follows: ops$tkyte@ORA10G> create table perm 2 ( x char(2000) , 3 y char(2000) , 4 z char(2000) ) 5 / Table created. ops$tkyte@ORA10G> create global temporary table temp 2 ( x char(2000) , 3 y char(2000) , 4 z char(2000) ) 5 on commit preserve rows 6 / Table created. I set up a small stored procedure to allow me to perform arbitrary SQL and report the amount of redo generated by that SQL. I will use this routine to perform INSERTs, UPDATEs, and DELETEs against both the temporary and permanent tables: ops$tkyte@ORA10G> create or replace procedure do_sql( p_sql in varchar2 ) 2 as 3 l_start_redo number; 4 l_redo number; 5 begin 6 select v$mystat.value 7 into l_start_redo 8 from v$mystat, v$statname 9 where v$mystat.statistic# = v$statname.statistic# 10 and v$statname.name = 'redo size'; 11 12 execute immediate p_sql;
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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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■SETTING UP YOUR ENVIRONMENT xlii
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CHAPTER 1 ■ ■ ■ Developing Su
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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 93 (data from m
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CHAPTER 3 ■ FILES 95 dictionary t
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CHAPTER 4 ■ MEMORY STRUCTURES 117
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CHAPTER 8 ■ ■ ■ Transactions
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CHAPTER 11 ■ ■ ■ Indexes Inde
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CHAPTER 13 ■ ■ ■ Partitioning
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