How to measure undo at a session level

Question

Below is what are trying to do.
We are trying to implement Oracle's table block compression feature.
In doing so, in one of our testing we discovered that the session performing the DML (inserts) generated almost 30x undo.
We measured this undo by using below query (before the transaction commited).
SELECT a.sid, a.username, used_ublk, used_ublk*8192 bytes
FROM v$session a, v$transaction b
WHERE a.saddr = b.ses_addr
However, above is at a transaction level since it still not committed, we would lose this value once the transaction either committed or rolled back, for this reason, we are trying to find an equivalent statistic at a session level.
1. What we are trying to find it out whether if an equivalent session level statistic exist to measure the amount of undo generated?
2. Is the undo generated always in terms of "undo blocks?"
3. When querying v$statname for name like '%undo%' we came across several statistics, the closest one
undo change vector size -in bytes?
4. desc test_table;
Name Type
ID NUMBER
sql> insert into test_table values (1);
5. However when we run the query against:
SELECT s.username,sn.name, ss.value
FROM v$sesstat ss, v$session s, v$statname sn
WHERE ss.sid = s.sid
AND sn.statistic# = ss.statistic#
AND s.sid =204
AND sn.name ='undo change vector size'
SID USERNAME NAME BYTES
204 NP4 undo change vector size 492
6. Query against: v$transaction
SELECT a.sid, a.username, used_ublk, used_ublk*8192 bytes
FROM v$session a, v$transaction b
WHERE a.saddr = b.ses_addr
SID USED_UBLK BYTES
204 1 8192
What are trying to understand is:
1. How can we or what is the correct statistic to determine how many undo blocks were generated by particular session?
2. What is the statistic: undo change vector size? What does it really mean? or measure?

Answer 1

Any transaction that generates Undo will use Undo Blocks in multiples of 1 --- i.e. the minimum allocation on disk is 8KB.
Furthermore, an Undo_Rec does not translate to a Table Row. The Undo has to capture changes to Indexes, block splits, other actions. Multiple changes to the same table/index block may be collapsed into one undo record/block etc etc.
Therefore, a transaction that generated 492 bytes of Undo would use 8KB of undo space because that is the minimum allocation.
You need to test with larger transactions.
SQL> update P_10 set col_2='ABC2' where mod(col_1,10)=0;
250000 rows updated.
SQL>
SQL> @active_transactions
       SID    SERIAL# SPID         USERNAME     PROGRAM                       XIDUSN  USED_UBLK  USED_UREC
       143        542 17159        HEMANT       sqlplus@DG844 (TNS V1-V3)          6       5176     500000
Statistic : db block changes                                      1,009,903
Statistic : db block gets                                         1,469,623
Statistic : redo entries                                            502,507
Statistic : redo size                                           117,922,016
Statistic : undo change vector size                              41,000,368
Statistic : table scan blocks gotten                                 51,954
Statistic : table scan rows gotten                               10,075,245Hemant K Chitale