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CHAPTER 9 RETRIEVAL: SOME ADVANCED FEATURES
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Don t execute these four steps too quickly in a row. You should take some time in between the steps. Tip
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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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Listing 9-25 shows a first example of a flashback query. First, we select the current situation with a regular query. Then we use the AS OF TIMESTAMP option in the FROM clause to go back in time. 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 9-25. Flashback Example: AS OF Syntax SQL> select empno, ename, deptno, msal 2 from e; EMPNO -------7499 7521 7654 7698 7844 7900 ENAME DEPTNO MSAL -------- -------- -------ALLEN 30 1610 WARD 30 1260 MARTIN 30 1260 BLAKE 30 2860 TURNER 30 1510 JONES 30 810
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SQL> select empno, ename, deptno, msal 2 from e 3 AS OF TIMESTAMP to_timestamp('×tamp'); Enter value for 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
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CHAPTER 9 RETRIEVAL: SOME ADVANCED FEATURES
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14 rows selected. SQL> / Enter value for timestamp: 01-OCT-2004 10:53:42.000 from e * ERROR at line 2: ORA-01466: unable to read data - table definition has changed SQL> Of course, the timestamps to be used in Listing 9-25 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 9-24 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 9-25 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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VERSIONS BETWEEN
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In Listing 9-26, we go one step further, using the VERSIONS BETWEEN operator. Now we get the complete history of the rows that is, as far as the Oracle DBMS is able reconstruct them. Listing 9-26. Flashback Example: VERSIONS BETWEEN Syntax SQL> break on empno SQL> 2 3 4 5 6 7 select , , from where order empno, msal versions_starttime versions_endtime e versions between timestamp minvalue and maxvalue deptno = 10 by empno, versions_starttime nulls first; VERSIONS_STARTTIME VERSIONS_ENDTIME ------------------------- ------------------------01-OCT-2004 10:54:23.000 01-OCT-2004 10:54:23.000 01-OCT-2004 10:54:41.000 01-OCT-2004 10:54:41.000 01-OCT-2004 10:55:24.000 01-OCT-2004 10:55:24.000 01-OCT-2004 10:54:23.000 01-OCT-2004 10:54:23.000 01-OCT-2004 10:54:41.000 01-OCT-2004 10:54:41.000 01-OCT-2004 10:55:24.000 01-OCT-2004 10:55:24.000 01-OCT-2004 10:54:23.000 01-OCT-2004 10:54:23.000 01-OCT-2004 10:54:41.000
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EMPNO MSAL -------- -------7782 2450 2460 2440 2440 7839 5000 5010 4990 4990 7934 1300 1310
CHAPTER 9 RETRIEVAL: SOME ADVANCED FEATURES
1290 01-OCT-2004 10:54:41.000 1290 01-OCT-2004 10:55:24.000 12 rows selected. SQL>
01-OCT-2004 10:55:24.000
By using the VERSIONS BETWEEN operator in the FROM clause, you introduce several additional pseudo columns, such as VERSIONS_STARTTIME and VERSIONS_ENDTIME. You can use these pseudo columns in your queries. By using the correct ORDER BY clause (watch the NULLS FIRST clause in Listing 9-26), you get a complete historical overview. You don t see a start time for the three oldest salary values because you created the rows too long ago, and you don t see an end time for the last value because it is the current salary value.
FLASHBACK TABLE
In 7, you learned that you can rescue an inadvertently dropped table from the recycle bin with the FLASHBACK TABLE command. Listing 9-27 shows another example of this usage. Listing 9-27. Using FLASHBACK TABLE ... TO BEFORE DROP SQL> drop table e; Table dropped. SQL> flashback table e to before drop; Flashback complete. SQL> select * from e; EMPNO -----7499 7521 7654 7698 7844 7900 SQL> You can go back to any point in time with the FLASHBACK TABLE command, as you can see in Listing 9-28. Note the following important difference: Listings 9-25 and 9-26 show queries against table E where you go back in time, but the FLASHBACK TABLE example in Listing 9-28 changes the database and restores table E to a given point in time. ENAME -------ALLEN WARD MARTIN BLAKE TURNER JONES INIT ----JAM TF P R JJ R JOB MGR BDATE MSAL COMM DEPTNO -------- ------ ----------- ------ ------ -----SALESREP 7698 20-FEB-1961 1610 300 30 SALESREP 7698 22-FEB-1962 1260 500 30 SALESREP 7698 28-SEP-1956 1260 1400 30 MANAGER 7839 01-NOV-1963 2860 30 SALESREP 7698 28-SEP-1968 1510 0 30 ADMIN 7698 03-DEC-1969 810 30
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