RE: Undo Generation
- From: "Mark W. Farnham" <mwf@xxxxxxxx>
- To: <oracle.tutorials@xxxxxxxxx>, "'oracle-l'" <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 28 Jun 2007 15:12:18 -0400
The delete has to store undo information for all the columns in each row.
The update only has to store undo information for the column you changed.
That's the short answer.
_____
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of DBA Deepak
Sent: Thursday, June 28, 2007 1:44 PM
To: oracle-l
Subject: Undo Generation
Hi Gurus,
Have a simple doubt. I did an experiment to know which DML statement
generates the Most/Least amount of UNDO for the same no. of records.
Steps:
===========================================================
SQL> conn scott/tiger
Connected.
SQL> create table myemp as select * from emp;
Table created.
SQL> delete myemp;
14 rows deleted.
SQL> select used_ublk,used_urec from v$session s,v$transaction t where
s.taddr=t.addr and s.username='SCOTT';
USED_UBLK USED_UREC
---------- ----------
2 14
SQL> rollback;
Rollback complete.
SQL> update myemp set sal=sal*1.1;
14 rows updated.
USED_UBLK USED_UREC
---------- ----------
1 9
SQL> rollback;
Rollback complete.
SQL> insert into myemp select * from emp;
14 rows created.
USED_UBLK USED_UREC
---------- ----------
1 1
SQL> rollback;
Rollback complete.
=================================================================
Please note that there was only one session connected to the database as
SCOTT user.
My question is why the do the UNDO generated for DELTE and UPDATE DML
statements differ? Because Oracle has to keep undo information of the same
no. of records in both the cases.
Please help me on this.
--
Regards,
Deepak
Oracle DBA
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