Redo and Undo - Oracle Database Transactions and Locking Revealed - page 110

The blocks that our INSERT caused to be added under the table's high-water mark (HWM) are still
there—formatted, but empty. Our full scan had to read them to see if they contained any rows. Moreover, the first
time we ran the query, we observed zero I/Os. That was due to the default mode of table creation in Oracle 11 g
Release 2—using deferred segment creation. When we issued that CREATE TABLE , no storage, not a single extent, was
allocated. The segment creation was deferred until the INSERT took place, and when we rolled back, the segment
persisted. You can see this easily with a smaller example, I'll explicitly request deferred segment creation this time
although it is enabled by default in 11 g Release 2 and higher:
EODA@ORA12CR1> drop table t purge;
Table dropped.
EODA@ORA12CR1> create table t ( x int )
2 segment creation deferred;
Table created.
EODA@ORA12CR1> select extent_id, bytes, blocks
2 from user_extents
3 where segment_name = 'T'
4 order by extent_id;
no rows selected
EODA@ORA12CR1> insert into t(x) values (1);
1 row created.
EODA@ORA12CR1> rollback;
Rollback complete.
EODA@ORA12CR1> select extent_id, bytes, blocks
2 from user_extents
3 where segment_name = 'T'
4 order by extent_id;
EXTENT_ID BYTES BLOCKS
---------- ---------- ----------
0 65536 8
As you can see, after the table was initially created there was no allocated storage—no extents were used by this
table. Upon performing an INSERT , followed immediately by ROLLBACK , we can see the INSERT allocated storage—but
the ROLLBACK does not “release” it.
Those two things together—that the segment was actually created by the INSERT but not “uncreated” by the
ROLLBACK , and that the new formatted blocks created by the INSERT were scanned the second time around—show that
a rollback is a logical “put the database back the way it was” operation. The database will not be exactly the way it was,
just logically the same.
How Redo and Undo Work Together
Now let's take a look at how redo and undo work together in various scenarios. We will discuss, for example, what
happens during the processing of an INSERT with regard to redo and undo generation, and how Oracle uses this
information in the event of failures at various points in time.