Oracle Database Discussions

What data do you need to undo an insert? No data, just the rowid that was created.

What data do you need to undo a delete? The complete row.

SQL> truncate table q;
SQL> insert into q select * from dba_objects ;

Statistics
---------------------------------------------------
         62  recursive calls
       9017  db block gets
       2716  consistent gets
          3  physical reads
    7636776  redo size   
      64851  rows processed
      

SQL> delete from q;

Statistics
---------------------------------------------------
         22  recursive calls
      72934  db block gets
       1072  consistent gets
          0  physical reads
   25324064  redo size
      64851  rows processed

See difference in redo size.

But "complete row" is inserted also to data tablespace.

... and that at first glance is counterintuitive because the reverse argument should apply to redo, until you recognize that undo is also protected by redo.

Sorry, not clear. For example, 'Insert' inserts 100 Mbytes to data segments. 'Delete' just moves this data into undo. Why redo is so different?

Because the operations that change both the undo segments and data segments that are also recorded in redo.

Go back and reread the concepts guide (several times) and understand the beautifully elegant way that the undo mechanism provides read consistency, rollback and statement level flashback and the redo mechanism gives recovery capabilities against software and hardware failure.

The key thing is that data and undo segments use the same recovery mechanism, the redo.

 'Delete' just moves this data into undo

No, both, Redo and undo are generated with insert and delete. Redo and Undo, are blocks and works equally.

The undo generated will be enough information to make the data "go away", the redo generated will be enough information to make the insert "happen again".

In your example, imagine that you create the new table inserting data from other table. Your table haven't anything "before" so undo is less than redo. When you delete the table, you have info "before" and "data to save" for "re-write" the table.

The delete makes more undo information because it has "before" data. That's why it need more time to finish.


from:
http://www.google.es/url?sa=t&rct=j&q=insert%20redo&source=web&cd=2&cad=rja&ved=0CD4QFjAB&url=http%3A%2F%2Fasktom.oracle.com%2Fpls%2Fapex%2Fz%3Fp_url%3DASKTOM.download_file%253Fp_file%253D1435150530862588762%26p_cat%3Dundo_redo.pdf%26p_company%3D822925097021874&ei=45wsUcDpKIuJhQfYroDQDQ&usg=AFQjCNFtDLW6XvP2aDN8zDCPki92UcK95A&bvm=bv.42965579,d.ZG4

Good document, but it doesn't explain in detail.
For example, 'insert' inserts 100Mb into data blocks. Undo size for this is small, assume, 1Mb. So total redo will be about 101Mb.
'Delete' just copies these 100Mb blocks into undo segments and marks data blocks as free. So total redo should be also about 100Mb ?! But it is several times more! Why?

1.When you insert rows, database populates data blocks. In this case size of redo will be about size of inserted blocks plus some small amount of undo blocks.
2.When you delete rows, database does not just copy original rows to undo, but creates undo records. Undo record for delete contains before images for all columns and is larger in size that usual data record. Therefore total amount of undo blocks will be more than total data blocks.
3.When you update rows, redo will include undo records and new data. The more columns are updated, the larger size of undo record and more new data. Therefore update operation can even generate more redo than delete. Updating all columns will generate almost the same redo as for delete + insert.