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CHAPTER 9 REDO AND UNDO
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every 1MB or so. When it came to the COMMIT, there wasn t much left to do not much more than when we created ten rows of data. You should expect to see similar (but not exactly the same) results, regardless of the amount of redo generated.
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What Does a ROLLBACK Do
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By changing the COMMIT to ROLLBACK, we can expect a totally different result. The time to roll back is definitely a function of the amount of data modified. I changed the script developed in the previous section to perform a ROLLBACK instead (simply change the COMMIT to ROLLBACK) and the timings are very different. Look at the results now: ops$tkyte%ORA11GR2> declare 2 l_redo number; 3 l_cpu number; 4 l_ela number; 5 begin 6 dbms_output.put_line 7 ( '-' || ' Rows' || ' Redo' || 8 ' CPU' || ' Elapsed' ); 9 for i in 1 .. 6 10 loop 11 l_redo := get_stat_val( 'redo size' ); 12 insert into t select * from big_table where rownum <= power(10,i); 13 l_cpu := dbms_utility.get_cpu_time; 14 l_ela := dbms_utility.get_time; 15 --commit work write wait; 16 rollback; 17 dbms_output.put_line 18 ( '-' || 19 to_char( power( 10, i ), '9,999,999') || 20 to_char( (get_stat_val('redo size')-l_redo), '999,999,999' ) || 21 to_char( (dbms_utility.get_cpu_time-l_cpu), '999,999' ) || 22 to_char( (dbms_utility.get_time-l_ela), '999,999' ) ); 23 end loop; 24 end; 25 / Rows Redo CPU Elapsed 10 3,036 0 0 100 10,632 2 0 1,000 120,532 0 0 10,000 1,212,540 1 1 100,000 12,973,208 9 10 - 1,000,000 130,473,232 124 980 PL/SQL procedure successfully completed. This difference in CPU and Elapsed timings is to be expected, as a ROLLBACK has to physically undo the work we ve done. Similar to a COMMIT, a series of operations must be performed. Before we even get to the ROLLBACK, the database has already done a lot of work. To recap, the following would have happened: Undo segment records have been generated in the SGA. Modified data blocks have been generated in the SGA.
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CHAPTER 9 REDO AND UNDO
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A buffered redo log for the preceding two items has been generated in the SGA. Depending on the size of the preceding three items and the amount of time spent, some combination of the previous data may be flushed onto disk already. All locks have been acquired.
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When we ROLLBACK, We undo all of the changes made. This is accomplished by reading the data back from the undo segment and, in effect, reversing our operation and then marking the undo entry as applied. If we inserted a row, a ROLLBACK will delete it. If we updated a row, a rollback will reverse the update. If we deleted a row, a rollback will re-insert it again. All locks held by our session are released, and everyone who was enqueued waiting on locks we held will be released.
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A COMMIT, on the other hand, just flushes any remaining data in the redo log buffers. It does very little work compared to a ROLLBACK. The point here is that you don t want to roll back unless you have to. It is expensive since you spend a lot of time doing the work, and you ll also spend a lot of time undoing the work. Don t do work unless you re sure you are going to want to COMMIT it. This sounds like common sense of course I wouldn t do all of the work unless I wanted to COMMIT it. However, I ve often seen a developer use a real table as a temporary table, fill it up with data, report on it, and then roll back to get rid of the temporary data. Later we ll talk about true temporary tables and how to avoid this issue.
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