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RETRIEVAL: SOME ADVANCED FEATURES
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9.7 Flashback Features
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This section covers some Oracle-specific extensions of the SQL language. Although they might appear slightly off topic, the flashback features are simply too valuable to remain uncovered in this book. In 6, we talked about the concept of read consistency. Read consistency means that your SQL statements always get a consistent view of the data, regardless of what other database users or applications do with the same data at the same time. The Oracle DBMS provides a snapshot of the data at the point in time when the statement execution started. In the same chapter, you also saw that you can change your session to be READ ONLY, so that your query results depend on the data as it was at the beginning of your session. The Oracle DBMS has its methods to achieve this, without using any locking techniques affecting other database users or applications. How this is done is irrelevant for this book. This section shows some interesting ways to use the same technique, by stating explicitly in your queries that you want to go back in time.
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Note The flashback query feature may need some configuration efforts before you can use it. This is the task of a database administrator. Therefore, it is not covered in this book. See the Oracle documentation for more details.
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Before we start our flashback query experiments, we first create a temporary copy of the EMPLOYEES table, as shown in Listing 9-31. (The listing is generated using SQL*Plus). This allows us to perform various experiments without destroying the contents of the real EMPLOYEES table. We also change the NLS_TIMESTAMP_FORMAT parameter with the ALTER SESSION command, to influence how timestamp values are displayed on the screen. Listing 9-31. Preparing for the Flashback Examples SQL> create table e as select * from employees; Table created. SQL> alter session set nls_timestamp_format='DD-MON-YYYY HH24:MI:SS.FF3'; Session altered. SQL> select localtimestamp as table_created from dual; TABLE_CREATED -----------------------------------------------------01-OCT-2004 10:53:42.746 SQL> update e set msal = msal + 10; 14 rows updated. SQL> commit; Commit complete.
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SQL> select localtimestamp as after_update_1 from dual; AFTER_UPDATE_1 ------------------------------------------------------01-OCT-2004 10:54:26.138 SQL> update e set msal = msal - 20 where 3 rows updated. SQL> commit; Commit complete. deptno = 10;
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SQL> select localtimestamp as after_update_2 from dual; AFTER_UPDATE_2 ------------------------------------------------------01-OCT-2004 10:54:42.602 SQL> delete from e where deptno <= 20; 8 rows deleted. SQL> commit; Commit complete. SQL> select localtimestamp as now from dual; NOW ------------------------------------------------------01-OCT-2004 10:55:25.623 SQL>
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Tip Don t execute these four steps too quickly in a row. You should take some time in between the steps. This makes it much easier during your experiments to go back to a specific point in time.
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AS OF
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Listings 9-32 to 9-34 show a first example of a flashback query. First, we select the current situation with a regular query (Listing 9-32). Then we use the AS OF TIMESTAMP option in the FROM clause to go back in time (Listing 9-33). Finally, we look at what happens when you try to go back in time beyond the amount of historical data that Oracle maintains (Listing 9-34). As in examples in earlier chapters, we use the SQL*Plus ampersand (&) substitution trick, which allows us to repeat the query conveniently with different timestamp values. Listing 9-32. Evaluating the current situation select empno, ename, deptno, msal from e; EMPNO ENAME DEPTNO MSAL -------- -------- -------- -------7499 ALLEN 30 1610
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7521 7654 7698 7844 7900
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30 30 30 30 30
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Listing 9-33. Querying as of some point in the past select empno, ename, deptno, msal from e AS OF TIMESTAMP to_timestamp('01-OCT-2004 10:53:47.000'); EMPNO -------7369 7499 7521 7566 7654 7698 7782 7788 7839 7844 7876 7900 7902 7934 ENAME DEPTNO MSAL -------- -------- -------SMITH 20 800 ALLEN 30 1600 WARD 30 1250 JONES 20 2975 MARTIN 30 1250 BLAKE 30 2850 CLARK 10 2450 SCOTT 20 3000 KING 10 5000 TURNER 30 1500 ADAMS 20 1100 JONES 30 800 FORD 20 3000 MILLER 10 1300
Listing 9-34. Querying for a point too far back in time select empno, ename, deptno, msal from e AS OF TIMESTAMP to_timestamp('01-OCT-2004 10:53:42.000'); * ERROR at line 2: ORA-01466: unable to read data - table definition has changed Of course, the timestamps to be used in Listing 9-32 depend on the timing of your experiments. Choose appropriate timestamps if you want to test these statements yourself. If you executed the steps of Listing 9-31 with some decent time intervals (as suggested), you have enough appropriate candidate values to play with. The Oracle error message at the bottom of Listing 9-32 indicates that this query is trying to go back too far in time. In this case, table E didn t even exist. Data definition changes (ALTER TABLE E ...) may also prohibit flashback queries, as suggested by the error message text.
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