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CHAPTER 6 LOCKING AND LATCHING
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ops$tkyte%ORA11GR2> update dept 2 set dname = initcap(:dname), 3 last_mod = systimestamp 4 where deptno = :deptno 5 and last_mod = to_timestamp_tz(:last_mod, 'DD-MON-YYYY HH.MI.SSXFF AM TZR' ); 1 row updated. As you can see, one row was updated, the row of interest. We updated the row by primary key (DEPTNO) and verified that the LAST_MOD column had not been modified by any other session between the time we read it first and the time we did the update. If we were to try to update that same record again, using the same logic but without retrieving the new LAST_MOD value, we would observe the following: ops$tkyte%ORA11GR2> update dept 2 set dname = upper(:dname), 3 last_mod = systimestamp 4 where deptno = :deptno 5 and last_mod = to_timestamp_tz(:last_mod, 'DD-MON-YYYY HH.MI.SSXFF AM TZR' ); 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 that the data has been changed in the database, based on the fact that no rows were modified 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 also conform to these rules. There are many chances to miss a spot in the application code and thus not have this field properly used. So, if the application code itself isn t responsible for maintaining this LAST_MOD field, then I believe that the application shouldn t be responsible for checking this LAST_MOD field either (if it can do the check, it can certainly do the update!). So, in this case, I suggest encapsulating the update logic in a stored procedure and not allowing the application to update the table directly at all. If it cannot be trusted to maintain the value in this field, then it cannot be trusted to check it properly either. So, the stored procedure would take as inputs the bind variables we used in the previous updates and do exactly the same update. Upon detecting that zero rows were updated, the stored procedure could raise an exception back to the client to let the client know the update had, in effect, failed. An alternate implementation uses a trigger to maintain this LAST_MOD field, but for something as simple as this, my recommendation is to avoid the trigger and let the DML take care of it. Triggers introduce a measurable amount of overhead, and in this case they would be unnecessary. Furthermore, the trigger would not be able to confirm that the row has not been modified (it would only be able to supply the value for LAST_MOD, not check it during the update!), hence the application has to be made painfully aware of this column and how to properly use it. So the trigger is not by itself sufficient.
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This is very similar to the previous version column method, but it uses the base data itself to compute a virtual version column. I ll quote the Oracle 11g Release 2 PL/SQL Supplied Packages Guide (before showing how to use one of the supplied packages!) to help explain the goal and concepts behind a checksum or hash function:
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