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

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CHAPTER 6 ■ LOCKING AND LATCHING 195 ops$tkyte@ORA10G> begin 2 for x in ( select deptno, dname, loc 3 from dept 4 where deptno = 10 5 for update nowait ) 6 loop 7 if ( hextoraw( 'C44F7052661CE945D385D5C3F911E70FA99407A6' ) 8 dbms_crypto.hash 9 ( utl_raw.cast_to_raw(x.deptno||'/'||x.dname||'/'||x.loc), 10 dbms_crypto.hash_sh1 ) ) 11 then 12 raise_application_error(-20001, 'Row was modified' ); 13 end if; 14 end loop; 15 update dept 16 set dname = lower(dname) 17 where deptno = 10; 18 commit; 19 end; 20 / PL/SQL procedure successfully completed. Upon requerying that data and computing the hash again after the update, we can see that the hash value is very different. If someone had modified the row before we did, our hash values would not have compared: ops$tkyte@ORA10G> begin 2 for x in ( select deptno, dname, loc 3 from dept 4 where deptno = 10 ) 5 loop 6 dbms_output.put_line( 'Dname: ' || x.dname ); 7 dbms_output.put_line( 'Loc: ' || x.loc ); 8 dbms_output.put_line( 'Hash: ' || 9 dbms_crypto.hash 10 ( utl_raw.cast_to_raw(x.deptno||'/'||x.dname||'/'||x.loc), 11 dbms_crypto.hash_sh1 ) ); 12 end loop; 13 end; 14 / Dname: accounting Loc: NEW YORK Hash: F3DE485922D44DF598C2CEBC34C27DD2216FB90F PL/SQL procedure successfully completed.
196 CHAPTER 6 ■ LOCKING AND LATCHING This example showed how to implement optimistic locking with a hash or checksum. You should bear in mind that computing a hash or checksum is a somewhat CPU-intensive operation—it is computationally expensive. On a system where CPU is a scarce resource, you must take this fact into consideration. However, this approach is much more “network-friendly,” as the transmission of a relatively small hash instead of a before and after image of the row (to compare column by column) over the network will consume much less of that resource. Our last example will use a new Oracle 10g function, ORA_ROWSCN, which is small in size like a hash, but not CPU intensive to compute. Optimistic Locking Using ORA_ROWSCN Starting with Oracle 10g Release 1, you have the option to use the built-in ORA_ROWSCN function. It works very much like the version column technique described previously, but it can be performed automatically by Oracle—you need no extra column in the table and no extra update/maintenance code to update this value. ORA_ROWSCN is based on the internal Oracle system clock, the SCN. Every time you commit in Oracle, the SCN advances (other things can advance it as well, but it only advances; it never goes back). The concept is identical to the previous methods in that you retrieve ORA_ROWSCN upon data retrieval, and you verify it has not changed when you go to update. The only reason I give it more than passing mention is that unless you created the table to support the maintenance of ORA_ROWSCN at the row level, it is maintained at the block level. That is, by default many rows on a single block will share the same ORA_ROWSCN value. If you update a row on a block with 50 other rows, then they will all have their ORA_ROWSCN advanced as well. This would almost certainly lead to many false positives, whereby you believe a row was modified that in fact was not. Therefore, you need to be aware of this fact and understand how to change the behavior. To see the behavior and then change it, we’ll use the small DEPT table again: ops$tkyte@ORA10G> create table dept 2 (deptno, dname, loc, data, 3 constraint dept_pk primary key(deptno) 4 ) 5 as 6 select deptno, dname, loc, rpad('*',3500,'*') 7 from scott.dept; Table created. Now we can inspect what block each row is on (it is safe to assume in this case they are in the same file, so a common block number indicates they are on the same block). I was using an 8KB block size with a row width of about 3,550 bytes, so I am expecting there to be two rows per block for this example: ops$tkyte@ORA10G> select deptno, dname, 2 dbms_rowid.rowid_block_number(rowid) blockno, 3 ora_rowscn 4 from dept;
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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 xxiii Next up are t
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