Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 9 ■ REDO AND UNDO
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.