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

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CHAPTER 9 ■ REDO AND UNDO 309 data dictionary operations will be logged regardless of the logging mode. The amount of redo generated can be significantly less. For this example of the NOLOGGING clause, I ran the following in a database running in ARCHIVELOG mode: ops$tkyte@ORA10G> select log_mode from v$database; LOG_MODE ------------ ARCHIVELOG ops$tkyte@ORA10G> @mystat "redo size" ops$tkyte@ORA10G> set echo off NAME VALUE ---------- ---------- redo size 5846068 ops$tkyte@ORA10G> create table t 2 as 3 select * from all_objects; Table created. ops$tkyte@ORA10G> @mystat2 ops$tkyte@ORA10G> set echo off NAME V DIFF ---------- ---------- ---------------- redo size 11454472 5,608,404 That CREATE TABLE generated about 5.5MB of redo information. We’ll drop and re-create the table, in NOLOGGING mode this time: ops$tkyte@ORA10G> drop table t; Table dropped. ops$tkyte@ORA10G> @mystat "redo size" ops$tkyte@ORA10G> set echo off NAME VALUE ---------- ---------- redo size 11459508 ops$tkyte@ORA10G> create table t 2 NOLOGGING 3 as 4 select * from all_objects; Table created.
310 CHAPTER 9 ■ REDO AND UNDO ops$tkyte@ORA10G> @mystat2 ops$tkyte@ORA10G> set echo off NAME V DIFF ---------- ---------- ---------------- redo size 11540676 81,168 This time, there is only 80KB of redo generated. As you can see, this makes a tremendous difference—5.5MB of redo versus 80KB. The 5.5MB is the actual table data itself; it was written directly to disk, with no redo log generated for it. If you test this on a NOARCHIVELOG mode database, you will not see any differences. The CREATE TABLE will not be logged, with the exception of the data dictionary modifications, in a NOARCHIVELOG mode database. If you would like to see the difference on a NOARCHIVELOG mode database, you can replace the DROP TABLE and CREATE TABLE with DROP INDEX and CREATE INDEX on table T. These operations are logged by default, regardless of the mode in which the database is running. This example also points out a valuable tip: test your system in the mode it will be run in production, as the behavior may be different. Your production system will be running in ARCHIVELOG mode; if you perform lots of operations that generate redo in this mode, but not in NOARCHIVELOG mode, you’ll want to discover this during testing, not during rollout to the users! Of course, it is now obvious that you will do everything you can with NOLOGGING, right? In fact, the answer is a resounding no. You must use this mode very carefully, and only after discussing the issues with the person in charge of backup and recovery. Let’s say you create this table and it is now part of your application (e.g., you used a CREATE TABLE AS SELECT NOLOGGING as part of an upgrade script). Your users modify this table over the course of the day. That night, the disk that the table is on fails. “No problem,” the DBA says. “We are running in ARCHIVELOG mode, and we can perform media recovery.” The problem is, however, that the initially created table, since it was not logged, is not recoverable from the archived redo log. This table is unrecoverable and this brings out the most important point about NOLOGGING operations: they must be coordinated with your DBA and the system as a whole. If you use them and others are not aware of that fact, you may compromise the ability of your DBA to recover your database fully after a media failure. They must be used judiciously and carefully. The important things to note about NOLOGGING operations are as follows: • Some amount of redo will be generated, as a matter of fact. This redo is to protect the data dictionary. There is no avoiding this at all. It could be of a significantly lesser amount than before, but there will be some. • NOLOGGING does not prevent redo from being generated by all subsequent operations. In the preceding example, I did not create a table that is never logged. Only the single, individual operation of creating the table was not logged. All subsequent “normal” operations such as INSERTs, UPDATEs, and DELETEs will be logged. Other special operations, such as a direct path load using SQL*Loader, or a direct path insert using the INSERT /*+ APPEND */ syntax, will not be logged (unless and until you ALTER the table and enable full logging again). In general, however, the operations your application performs against this table will be logged.
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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 SUCCESSFUL
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CHAPTER 3 ■ FILES 89 A Brief Revi
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CHAPTER 4 ■ MEMORY STRUCTURES 117
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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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