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This was a very artificial example just to show how it happens in a reliable manner. My UPDATE statement was generating undo. I had a very small undo tablespace to play with (2MB in size). I wrapped around in my undo segments many times, since they are used in a circular fashion. Every time I committed, I allowed Oracle to overwrite the undo data I generated. Eventually, I needed some piece of data that I had generated, but it no longer existed and I received the ORA-01555 error. You would be right to point out that in this case, if I had not committed on line 10 in the preceding example, then I would have received the following error: begin * ERROR at line 1: ORA-30036: unable to extend segment by 8 in undo tablespace 'UNDO_SMALL' ORA-06512: at line 6 The major differences between the two errors are as follows: The ORA-01555 example left my update in a totally unknown state. Some of the work had been done; some had not. There is absolutely nothing I can do to avoid the ORA-01555, given that I committed in the cursor FOR loop. The ORA-30036 error can be avoided by allocating appropriate resources in the system. This error is avoidable by correct sizing; the first error is not. Further, even if I don t avoid this error, at least the update is rolled back and the database is left in a known, consistent state I m not left halfway through some large update. The bottom line here is that you cannot save on undo space by committing frequently you need that undo. I was in a single-user system when I received the ORA-01555 error. It takes only one session to cause that error, and many times even in real life it is a single session causing its own ORA-01555 errors. Developers and DBAs need to work together to size these segments adequately for the jobs that need to be done. There can be no short-changing here. You must discover, through analysis of your system, what your biggest transactions are and size appropriately for them. The dynamic performance view V$UNDOSTAT can be very useful to monitor the amount of undo you are generating and the duration of your longest running queries. Many people consider things like temp, undo, and redo as overhead things to allocate as little storage to as possible. This is reminiscent of a problem the computer industry had on January 1, 2000, which was all caused by trying to save 2 bytes in a date field. These components of the database are not overhead, but rather are key components of the system. They must be sized appropriately (not too big and not too small).
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The most serious problem with the commit before the logical transaction is over approach is the fact that it frequently leaves your database in an unknown state if the UPDATE fails halfway through. Unless you planned for this ahead of time, it is very hard to restart the failed process, allowing it to pick up where it left off. For example, say we were not applying the LOWER() function to the column, as in the previous example, but rather some other function of the column such as the following:
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last_ddl_time = last_ddl_time + 1; If we halted the UPDATE loop partway through, how would we restart it We could not just rerun it, as we would end up adding 2 to some dates, and 1 to others. If we fail again, we would add 3 to some, 2 to others, 1 to the rest, and so on. We need yet more complex logic some way to partition the data. For example, we could process every OBJECT_NAME that starts with A, and then B, and so on: ops$tkyte@ORA10G> create table to_do 2 as 3 select distinct substr( object_name, 1,1 ) first_char 4 from T 5 / Table created. ops$tkyte@ORA10G> begin 2 for x in ( select * from to_do ) 3 loop 4 update t set last_ddl_time = last_ddl_time+1 5 where object_name like x.first_char || '%'; 6 7 dbms_output.put_line( sql%rowcount || ' rows updated' ); 8 delete from to_do where first_char = x.first_char; 9 10 commit; 11 end loop; 12 end; 13 / 22257 rows updated 1167 rows updated 135 rows updated 1139 rows updated 2993 rows updated 691 rows updated ... 2810 rows updated 6 rows updated 10 rows updated 2849 rows updated 1 rows updated 2 rows updated 7 rows updated PL/SQL procedure successfully completed. Now, we could restart this process if it fails, since we would not process any object name that had already been processed successfully. The problem with this approach, however, is that unless we have some attribute that evenly partitions the data, we will end up having a
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