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

CHAPTER 8 ■ TRANSACTIONS 269
ops$tkyte@ORA10G> exec dbms_stats.gather_table_stats( user, 'T', cascade=>true );
PL/SQL procedure successfully completed.
I then created a very small undo tablespace and altered the system to use it. Note that by
setting AUTOEXTEND off, I have limited the size of all UNDO to be 2MB or less in this system:
ops$tkyte@ORA10G> create undo tablespace undo_small
2 datafile size 2m
3 autoextend off
4 /
Tablespace created.
ops$tkyte@ORA10G> alter system set undo_tablespace = undo_small;
System altered.
Now, with only the small undo tablespace in use, I ran this block of code to do the UPDATE:
ops$tkyte@ORA10G> begin
2 for x in ( select /*+ INDEX(t t_idx) */ rowid rid, object_name, rownum r
3 from t
4 where object_name > ' ' )
5 loop
6 update t
7 set object_name = lower(x.object_name)
8 where rowid = x.rid;
9 if ( mod(x.r,100) = 0 ) then
10 commit;
11 end if;
12 end loop;
13 commit;
14 end;
15 /
begin
*
ERROR at line 1:
ORA-01555: snapshot too old: rollback segment number with name "" too small
ORA-06512: at line 2
I get the error. I should point out that I added an index hint to the query and a WHERE
clause to make sure I was reading the table randomly (together, they caused the cost-based
optimizer to read the table “sorted” by the index key). When we process a table via an index,
we will tend to read a block for a single row, and then the next row we want will be on a different
block. Ultimately, we will process all of the rows on block 1, just not all at the same time.
Block 1 might hold, say, the data for all rows with OBJECT_NAMEs starting with the letters A, M,
N, Q, and Z. So we would hit the block many times, since we are reading the data sorted by
OBJECT_NAME and presumably many OBJECT_NAMEs start with letters between A and M. Since we
are committing frequently and reusing undo space, we eventually revisit a block where we can
simply no longer roll back to the point in time our query began, and at that point we get the
error.