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CHAPTER 9 REDO AND UNDO
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That table will have lots of blocks as we get about six or seven rows per block using that big data field. Next, we ll create the small table that the many little transactions will modify: ops$tkyte@ORA10G> create table small ( x int, y char(500) ); Table created. ops$tkyte@ORA10G> insert into small select rownum, 'x' from all_users; 38 rows created. ops$tkyte@ORA10G> commit; Commit complete. ops$tkyte@ORA10G> exec dbms_stats.gather_table_stats( user, 'SMALL' ); PL/SQL procedure successfully completed. Now, we ll dirty up that big table. We have a very small undo tablespace, so we ll want to update as many blocks of this big table as possible, all while generating the least amount of undo possible. We ll be using a fancy UPDATE statement to do that. Basically, the following subquery is finding the first rowid of a row on every block. That subquery will return a rowid for each and every database block identifying a single row on it. We ll update that row, setting a VARCHAR2(1) field. This will let us update all of the blocks in the table (some 8,000 plus in the example), flooding the buffer cache with dirty blocks that will have to be written out (we have room for only 500 right now). We ll make sure we are using that small undo tablespace as well. To accomplish this and not exceed the capacity of our undo tablespace, we ll craft an UPDATE statement that will update just the first row on each block. The ROW_NUMBER() built-in analytic function is instrumental in this operation; it assigns the number 1 to the first row by database block in the table, which would be the single row on the block we would update: ops$tkyte@ORA10G> alter system set undo_tablespace = undo_small; System altered. ops$tkyte@ORA10G> update big 2 set temporary = temporary 3 where rowid in 4 ( 5 select r 6 from ( 7 select rowid r, row_number() over (partition by dbms_rowid.rowid_block_number(rowid) order by rowid) rn 8 from big 9 ) 10 where rn = 1 11 ) 12 / 8045 rows updated. ops$tkyte@ORA10G> commit; Commit complete.
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CHAPTER 9 REDO AND UNDO
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OK, so now we know that we have lots of dirty blocks on disk. We definitely wrote some of them out, but we just did not have the room to hold them all. Next, we opened a cursor, but did not yet fetch a single row. Remember, when we open the cursor, the result set is preordained, so even though Oracle did not actually process a row of data, the act of opening that result set fixed the point in time the results must be as of. Now since we will be fetching the data we just updated and committed, and we know no one else is modifying the data, we should be able to retrieve the rows without needing any undo at all. But that is where the delayed block cleanout rears its head. The transaction that modified these blocks is so new that Oracle will be obliged to verify that it committed before we begin, and if we overwrite that information (also stored in the undo tablespace), the query will fail. So, here is the opening of the cursor: ops$tkyte@ORA10G> variable x refcursor ops$tkyte@ORA10G> exec open :x for select * from big; PL/SQL procedure successfully completed. ops$tkyte@ORA10G> !./run.sh run.sh is a shell script. It simply fired off nine SQL*Plus sessions using a command: $ORACLE_HOME/bin/sqlplus / @test2 1 &
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where each SQL*Plus session was passed a different number (that was number 1; there was a 2, 3, and so on). The script test2.sql they each ran is as follows: begin for i in 1 .. 1000 loop update small set y = i where x= &1; commit; end loop; end; / exit So, we had nine sessions inside of a tight loop initiate many transactions. The run.sh script waited for the nine SQL*Plus sessions to complete their work, and then we returned to our session, the one with the open cursor. Upon attempting to print it out, we observe the following: ops$tkyte@ORA10G> print x ERROR: ORA-01555: snapshot too old: rollback segment number 23 with name "_SYSSMU23$" too small no rows selected As I said, the preceding is a rare case. It took a lot of conditions, all of which must exist simultaneously to occur. We needed blocks that were in need of a cleanout to exist, and these blocks are rare in Oracle8i and above. A DBMS_STATS call to collect statistics gets rid of them so the most common causes large mass updates and bulk loads should not be a concern, since the tables need to be analyzed after such operations anyway. Most transactions tend to
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