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The pessimistic locking method would be put into action the instant before a user modifies a value on the screen. For example, a row lock would be placed as soon as the user indicates his intention to perform an update on a specific row that he has selected and has visible on the screen (by clicking a button on the screen, say). That row lock would persist until the application applied the users' modifications to the row in the database and committed. Pessimistic locking is useful only in a stateful or connected environment that is, one where your application has a continual connection to the database and you are the only one using that connection for at least the life of your transaction. This was the prevalent way of doing things in the early to mid 1990s with client/server applications. Every application would get a direct connection to the database to be used solely by that application instance. This method of connecting, in a stateful fashion, has become less common (though it is not extinct), especially with the advent of application servers in the mid to late 1990s. Assuming you are using a stateful connection, you might have an application that queries the data without locking anything: scott%ORA11GR2> select empno, ename, sal from emp where deptno = 10; EMPNO ---------7782 7839 7934 ENAME SAL ---------- ---------CLARK 2450 KING 5000 MILLER 1300
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Eventually, the user picks a row she would like to update. Let s say in this case, she chooses to update the MILLER row. Our application will, at that point, (before the user makes any changes on the screen but after the row has been out of the database for a while) bind the values the user selected so we can query the database and make sure the data hasn t been changed yet. In SQL*Plus, to simulate the bind calls the application would make, we can issue the following: scott%ORA11GR2> scott%ORA11GR2> scott%ORA11GR2> scott%ORA11GR2> variable empno number variable ename varchar2(20) variable sal number exec :empno := 7934; :ename := 'MILLER'; :sal := 1300;
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PL/SQL procedure successfully completed.
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CHAPTER 6 LOCKING AND LATCHING
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Now in addition to simply querying the values and verifying that they have not been changed, we are going to lock the row using FOR UPDATE NOWAIT. The application will execute the following query: scott%ORA11GR2> select empno, ename, sal 2 from emp 3 where empno = :empno 4 and decode( ename, :ename, 1 ) = 1 5 and decode( sal, :sal, 1 ) = 1 6 for update nowait 7 / EMPNO ENAME SAL ---------- ---------- ---------7934 MILLER 1300
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Note Why did we use decode(
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column, :bind_variable, 1 ) = 1 It is simply a shorthand way of
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expressing where (column = :bind_variable OR (column is NULL and :bind_variable is NULL) . You could code either approach, the decode() is just more compact in this case, and since NULL = NULL is never true (nor false!) in SQL, one of the two approaches would be necessary if either of the columns permitted NULLs.
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The application supplies values for the bind variables from the data on the screen (in this case 7934, MILLER, and 1300) and re-queries this same row from the database, this time locking the row against updates by other sessions; hence this approach is called pessimistic locking. We lock the row before we attempt to update because we doubt we are pessimistic that the row will remain unchanged otherwise. Since all tables should have a primary key (the preceding SELECT will retrieve at most one record since it includes the primary key, EMPNO) and primary keys should be immutable (we should never update them), we ll get one of three outcomes from this statement: If the underlying data has not changed, we will get our MILLER row back, and this row will be locked from updates (but not reads) by others. If another user is in the process of modifying that row, we will get an ORA-00054 resource busy error. We must wait for the other user to finish with it. If, in the time between selecting the data and indicating our intention to update, someone has already changed the row, then we will get zero rows back. That implies the data on our screen is stale. To avoid the lost update scenario previously described, the application needs to re-query and lock the data before allowing the end user to modify it. With pessimistic locking in place, when User2 attempts to update the telephone field, the application would now recognize that the address field had been changed and would re-query the data. Thus, User2 would not overwrite User1 s change with the old data in that field.
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Once we have locked the row successfully, the application will bind the new values, issue the update, and commit the changes:
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