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

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CHAPTER 6 ■ LOCKING AND LATCHING 197 DEPTNO DNAME BLOCKNO ORA_ROWSCN ---------- -------------- ---------- ---------- 10 ACCOUNTING 20972 34676029 20 RESEARCH 20972 34676029 30 SALES 20973 34676029 40 OPERATIONS 20973 34676029 And sure enough, that is what we observe in this case. So, let’s update the row where DEPTNO = 10 is on block 20972: ops$tkyte@ORA10G> update dept 2 set dname = lower(dname) 3 where deptno = 10; 1 row updated. ops$tkyte@ORA10G> commit; Commit complete. What we’ll observe next shows the consequences of ORA_ROWSCN being tracked at the block level. We modified and committed the changes to a single row, but the ORA_ROWSCN values of both of the rows on block 20972 have been advanced: ops$tkyte@ORA10G> select deptno, dname, 2 dbms_rowid.rowid_block_number(rowid) blockno, 3 ora_rowscn 4 from dept; DEPTNO DNAME BLOCKNO ORA_ROWSCN ---------- -------------- ---------- ---------- 10 accounting 20972 34676046 20 RESEARCH 20972 34676046 30 SALES 20973 34676029 40 OPERATIONS 20973 34676029 It would appear to anyone else that had read the DEPTNO=20 row that it had been modified, even though it was not. The rows on block 20973 are “safe”—we didn’t modify them, so they did not advance. However, if we were to update either of them, both would advance. So the question becomes how to modify this default behavior. Well, unfortunately, we have to re-create the segment with ROWDEPENDENCIES enabled. Row dependency tracking was added to the database with Oracle9i in support of advanced replication to allow for better parallel propagation of changes. Prior to Oracle 10g, its only use was in a replication environment, but starting in Oracle 10g we can use it to implement an effective optimistic locking technique with ORA_ROWSCN. It will add 6 bytes of overhead to each row (so it is not a space saver compared to the do-it-yourself version column) and that is, in fact, why it requires a table re-create and not just a simple ALTER TABLE: the physical block structure must be changed to accommodate this feature. Let’s rebuild our table to enable ROWDEPENDENCIES. We could use the online rebuild capabilities in DBMS_REDEFINITION (another supplied package) to do this, but for something so small, we’ll just start over:
198 CHAPTER 6 ■ LOCKING AND LATCHING ops$tkyte@ORA10G> drop table dept; Table dropped. ops$tkyte@ORA10G> create table dept 2 (deptno, dname, loc, data, 3 constraint dept_pk primary key(deptno) 4 ) 5 ROWDEPENDENCIES 6 as 7 select deptno, dname, loc, rpad('*',3500,'*') 8 from scott.dept; Table created. ops$tkyte@ORA10G> select deptno, dname, 2 dbms_rowid.rowid_block_number(rowid) blockno, 3 ora_rowscn 4 from dept; DEPTNO DNAME BLOCKNO ORA_ROWSCN ---------- -------------- ---------- ---------- 10 ACCOUNTING 21020 34676364 20 RESEARCH 21020 34676364 30 SALES 21021 34676364 40 OPERATIONS 21021 34676364 We’re back where we were before: four rows on two blocks, all having the same initial ORA_ROWSCN value. Now when we update DEPTNO=10 ops$tkyte@ORA10G> update dept 2 set dname = lower(dname) 3 where deptno = 10; 1 row updated. ops$tkyte@ORA10G> commit; Commit complete. we should observe the following upon querying the DEPT table: ops$tkyte@ORA10G> select deptno, dname, 2 dbms_rowid.rowid_block_number(rowid) blockno, 3 ora_rowscn 4 from dept; DEPTNO DNAME BLOCKNO ORA_ROWSCN ---------- -------------- ---------- ---------- 10 accounting 21020 34676381 20 RESEARCH 21020 34676364 30 SALES 21021 34676364 40 OPERATIONS 21021 34676364
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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 71 all operatio
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CHAPTER 3 ■ FILES 111 IMPDP, howe
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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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294 CHAPTER 9 ■ REDO AND UNDO row
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338 CHAPTER 10 ■ DATABASE TABLES
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CHAPTER 11 ■ ■ ■ Indexes Inde
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CHAPTER 11 ■ INDEXES 431 The data
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CHAPTER 11 ■ INDEXES 445 Indicate
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CHAPTER 11 ■ INDEXES 447 an index
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CHAPTER 11 ■ INDEXES 449 Table 11
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CHAPTER 13 ■ ■ ■ Partitioning
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