Skip to Main Content

Breadcrumb

Question and Answer

Tom Kyte

Thanks for the question, ayush.

Asked: November 11, 2012 - 3:17 am UTC

Last updated: November 19, 2012 - 9:05 am UTC

Version: 10.2.4

Viewed 10K+ times! This question is

You Asked

Hi Tom, How are you doing.

I have a generic question. During which DML operation ( Insert,update and delete)
is the most undo and redo generated and why.

Regards
Ayush

and Tom said...

it depends :)

all you have to do however is think about what happens during each operation under different conditions - then you can sort of guestimate what will happen.

during an insert - we have to generate undo equivalent to "delete+rowid"

during an update - we have to generate undo equivalent to "update this rowid to have these before image values" (undo = rowid + column before images)

during a delete - we have to generate undo equivalent to "insert at this rowid these values"

so, considering an unindexed table - IN GENERAL, a delete and update will generate the most and an insert the least. IN GENERAL this is true (but not always).


if you add indexes - everything can change. the insert might generate gobs of undo for the index maintenance - the update might not update any indexed columns and hence only generate undo for the table - and the delete might have to maintain all of the indexes generating tons of undo again.

so, it depends on the nature of the data, what you are inserting/updating/deleting..

same for redo. It depends on what is modified...

Rating

  (2 ratings)

Is this answer out of date? If it is, please let us know via a Comment

Comments

Ayush Agarwal, November 12, 2012 - 10:37 pm UTC


Excellent

Ayush Agarwal, November 15, 2012 - 12:41 am UTC

Hi Tom,

If it's possible, can you please provide example for the above issue. So that we can understand better.
Tom Kyte
November 19, 2012 - 9:05 am UTC

just perform a few operations and measure things....

here is an example with indexes...

ops$tkyte%ORA11GR2> create table t
  2  as
  3  select a.*
  4    from all_objects a
  5   where 1=0
  6  /

Table created.

ops$tkyte%ORA11GR2> create index t_idx1 on t(object_name);

Index created.

ops$tkyte%ORA11GR2> create index t_idx2 on t(object_type);

Index created.

ops$tkyte%ORA11GR2> create index t_idx3 on t(timestamp);

Index created.

ops$tkyte%ORA11GR2> 
ops$tkyte%ORA11GR2> connect /
Connected.
ops$tkyte%ORA11GR2> set autotrace on statistics
ops$tkyte%ORA11GR2> insert into t select a.* from all_objects a
  2  /

72911 rows created.


Statistics
----------------------------------------------------------
       1514  recursive calls
     224491  db block gets
      57503  consistent gets
        888  physical reads
   62639584  redo size
        902  bytes sent via SQL*Net to client
       1011  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
       1501  sorts (memory)
          0  sorts (disk)
      72911  rows processed

ops$tkyte%ORA11GR2> set autotrace off
ops$tkyte%ORA11GR2> @mystat "undo change"
ops$tkyte%ORA11GR2> set echo off

NAME                         VALUE
----------------------- ----------
undo change vector size   24025452

ops$tkyte%ORA11GR2> commit;

Commit complete.

ops$tkyte%ORA11GR2> 
ops$tkyte%ORA11GR2> connect /
Connected.
ops$tkyte%ORA11GR2> set autotrace on statistics
ops$tkyte%ORA11GR2> update t set object_id = object_id+1;

72911 rows updated.


Statistics
----------------------------------------------------------
         79  recursive calls
      78595  db block gets
       1324  consistent gets
        112  physical reads
   34121652  redo size
        902  bytes sent via SQL*Net to client
       1004  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
      72911  rows processed

ops$tkyte%ORA11GR2> set autotrace off
ops$tkyte%ORA11GR2> @mystat "undo change"
ops$tkyte%ORA11GR2> set echo off

NAME                         VALUE
----------------------- ----------
undo change vector size   22234080

ops$tkyte%ORA11GR2> commit;

Commit complete.

ops$tkyte%ORA11GR2> 
ops$tkyte%ORA11GR2> connect /
Connected.
ops$tkyte%ORA11GR2> set autotrace on statistics
ops$tkyte%ORA11GR2> delete from t;

72911 rows deleted.


Statistics
----------------------------------------------------------
        423  recursive calls
     599516  db block gets
       2751  consistent gets
        722  physical reads
   78043120  redo size
        902  bytes sent via SQL*Net to client
        981  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
      72911  rows processed

ops$tkyte%ORA11GR2> set autotrace off
ops$tkyte%ORA11GR2> @mystat "undo change"
ops$tkyte%ORA11GR2> set echo off

NAME                         VALUE
----------------------- ----------
undo change vector size   40230900

ops$tkyte%ORA11GR2> commit;

Commit complete.

more

Connor and Chris don't just spend all day on AskTOM. You can also catch regular content via Connor's blog and Chris's blog. Or if video is more your thing, check out Connor's latest video and Chris's latest video from their Youtube channels. And of course, keep up to date with AskTOM via the official twitter account.