Undo confusion

Question

Hi,
As part of my understanding of how undo works, could somebody explain to me the following as I'm quite confused on this.
1. create table a (b number, c varchar2(30));
2. insert into a values (1,'hallo');
3. Check how many undo records have been created:
select used_urec from v$session s, v$transaction t
where s.audsid=sys_context('userenv', 'sessionid') and
s.taddr = t.addr;
This show 1 undo record because insert stores 1 undo record for rowid ?
5. insert into a values (2,'foo'); this creates 2 undo records (because of accumlation
from the previous one?)
4. insert into a select rownum + 50000, object_name from all_objects where rownum <
51;
5. This creates 3 undo records even though 50 inserts have occurred. Why only 3?
Would appreciate any comments on this?
Note: the above example has been taken from:
http://www.adp-gmbh.ch/ora/concepts/undo.html

Answer 1

What's your Oracle version?
My test on 10gR1 is different than yours. There's a field for used undo blocks in v$transaction,
USED_UBLK
TEST@rman>  create table a (b number, c varchar2(30));
Table created.
TEST@rman> insert into a values (1,'hallo');
1 row created.
SQL>
  1  select used_ublk,used_urec from v$session s, v$transaction t
  2  where s.audsid=sys_context('userenv', 'sessionid') and
  3* s.taddr = t.addr
TEST@rman> /
USED_UBLK  USED_UREC
         1          1
TEST@rman>  insert into a select rownum + 50000, object_name from all_objects where rownum <51
  2  /
50 rows created.
TEST@rman>  select used_ublk,used_urec from v$session s, v$transaction t
  2  where s.audsid=sys_context('userenv', 'sessionid') and
  3   s.taddr = t.addr
  4  /
USED_UBLK  USED_UREC
         2         51==========================
On the safe side, I inserted another two batches, 50 and 1000, the resule is pretty constant,
TEST@rman> insert into a select rownum + 50000, object_name from all_objects where rownum <51;
50 rows created.
TEST@rman>  select count(*) from a;
  COUNT(*)
       101
TEST@rman> select used_ublk,used_urec from v$session s, v$transaction t
  2   where s.audsid=sys_context('userenv', 'sessionid') and
  3    s.taddr = t.addr
  4  /
USED_UBLK  USED_UREC
         2        101
TEST@rman> insert into a select rownum + 50000, object_name from all_objects where rownum <1001 ;
1000 rows created.
TEST@rman>  select count(*) from a;
  COUNT(*)
      1101
TEST@rman>  select used_ublk,used_urec from v$session s, v$transaction t
  2   where s.audsid=sys_context('userenv', 'sessionid') and
  3    s.taddr = t.addr
  4  /
USED_UBLK  USED_UREC
        11       1101