A free extent is a collection of continuous free blocks in
tablespace. When a segment is dropped its extents are
reallocated and are marked as free.

FREE EXTENT MEANS UNALLOCATED AREA OF A SEGMENT.

WHEN WE DELETE DATA FROM TABLE, EXTENT OF CORROSPONDING 
SEGMENT BECOMES FREE.

BUT IF WE ARE TRUNCATING A TABLE IT REMOVES EXTENTS ALSO.

The space used transaction entries and deleted records does
not become free immediately after completion due to delayed
cleanout.

Trailling nulls and length bytes are not stored.


Inserts of, updates to and deletes of rows as well as
columns larger than a single datablock, can cause
fragmentation an chained row pieces.

system tablespace is a main part of oracle database.All the 
database information is in it.it is created when database 
is created.
its managed by oracle server.dba cannot change its 
containts.

it contains the data dictionary objects which cannot be 
modified by dba with few exceptions like aud$

CREATE ROLLBACK SEGMENT rbs_one
   TABLESPACE rbs_ts
   STORAGE
   ( INITIAL 10K
     NEXT 10K
     MAXEXTENTS UNLIMITED )

if the original disk has been crashed, the data can be
recovered by using this method. just like raid levels

PCTFREE parameter

Querying the DBA_ROLLBACK_SEGS view
IN USE         - Rollback Segment is on-line.
AVAILABLE - Rollback Segment available but not on-line.
OFF-LINE  - Rollback Segment off-line
INVALID        - Rollback Segment Dropped.
NEEDS RECOVERY  - Contains data but need recovery or corupted.
PARTLY  AVAILABLE  - Contains data from an unresolved
transaction involving a distributed database.

Each block contains entries as follows
Fixied block header
Variable block header
Row Header,row date (multiple rows may exists)
PCTEREE (% of  free space for row updation in future)

Rollback  segment  dynamically  extent  to handle larger
transactions entry loads.

A  single  transaction may wipeout all avaliable free space
in the Rollback Segment Tablespace. This prevents other user
using Rollback segments.

it is a component of SGA. this is used to check the syntax
and the semantics of the query.

it is a part of shared pool which contains most recently 
used dictionary data like priviledges, objects, 
users,packages....

Process of having a copy of redo log files is called mirroring.

This  can be achieved by creating group of log files
together, so that LGWR will  automatically  writes  them to
all the members of the current on-line redo  log  group. If
any one group fails then database automatically switch over
to next group. It degrades performance

We  can  plan  the  storage  for  a  table  as how much
initial extents are required,  how  much can be extended
next, how much % should leave free for managing row
updations etc.,

We  should  depend  on  export/import  utility  for 
backup/recovery (fully reliable)

The tar command cannot be used for physical file backup,
instead we can use dd command which is less flexible and has
limited recoveries.

we can increase the size of the database block in the
parameter file (init.ora).

When Segment is dropped.
When Shrink (RBS only)
When truncated (TRUNCATE used with drop storage option)