Last SCN and last squence

hi,
on 10g R2 is there any query to see last SCN and last squence number ?
thank you.

You can find many interesting things in v$database. Just check...
SQL> desc v$database
Nazwa                                     WartoťŠ NULL? Typ
DBID                                               NUMBER
NAME                                               VARCHAR2(9)
CREATED                                            DATE
RESETLOGS_CHANGE#                                  NUMBER
RESETLOGS_TIME                                     DATE
PRIOR_RESETLOGS_CHANGE#                            NUMBER
PRIOR_RESETLOGS_TIME                               DATE
LOG_MODE                                           VARCHAR2(12)
CHECKPOINT_CHANGE#                                 NUMBER
ARCHIVE_CHANGE#                                    NUMBER
CONTROLFILE_TYPE                                   VARCHAR2(7)
CONTROLFILE_CREATED                                DATE
CONTROLFILE_SEQUENCE#                              NUMBER
CONTROLFILE_CHANGE#                                NUMBER
CONTROLFILE_TIME                                   DATE
OPEN_RESETLOGS                                     VARCHAR2(11)
VERSION_TIME                                       DATE
OPEN_MODE                                          VARCHAR2(10)
PROTECTION_MODE                                    VARCHAR2(20)
PROTECTION_LEVEL                                   VARCHAR2(20)
REMOTE_ARCHIVE                                     VARCHAR2(8)
ACTIVATION#                                        NUMBER
SWITCHOVER#                                        NUMBER
DATABASE_ROLE                                      VARCHAR2(16)
ARCHIVELOG_CHANGE#                                 NUMBER
ARCHIVELOG_COMPRESSION                             VARCHAR2(8)
SWITCHOVER_STATUS                                  VARCHAR2(20)
DATAGUARD_BROKER                                   VARCHAR2(8)
GUARD_STATUS                                       VARCHAR2(7)
SUPPLEMENTAL_LOG_DATA_MIN                          VARCHAR2(8)
SUPPLEMENTAL_LOG_DATA_PK                           VARCHAR2(3)
SUPPLEMENTAL_LOG_DATA_UI                           VARCHAR2(3)
FORCE_LOGGING                                      VARCHAR2(3)
PLATFORM_ID                                        NUMBER
PLATFORM_NAME                                      VARCHAR2(101)
RECOVERY_TARGET_INCARNATION#                       NUMBER
LAST_OPEN_INCARNATION#                             NUMBER
CURRENT_SCN                                        NUMBER
FLASHBACK_ON                                       VARCHAR2(18)
SUPPLEMENTAL_LOG_DATA_FK                           VARCHAR2(3)
SUPPLEMENTAL_LOG_DATA_ALL                          VARCHAR2(3)
DB_UNIQUE_NAME                                     VARCHAR2(30)
STANDBY_BECAME_PRIMARY_SCN                         NUMBER
FS_FAILOVER_STATUS                                 VARCHAR2(21)
FS_FAILOVER_CURRENT_TARGET                         VARCHAR2(30)
FS_FAILOVER_THRESHOLD                              NUMBER
FS_FAILOVER_OBSERVER_PRESENT                       VARCHAR2(7)
FS_FAILOVER_OBSERVER_HOST                          VARCHAR2(512)
SQL>

Similar Messages

Oralce streamsSystem Change Number (SCN) and capture process

Do we have get SCN before capture process is started ? If yes , from where does the replication is started .
WIll replication process startright from the time when SCN is captured
OR
will replicaion start from the time when the capture process is started
Edited by: [email protected] on Mar 26, 2009 6:04 PM

I am trying to setup oracle streams to enable replication for a set of tables.
One of the step as per the doc is is to setup/get SCN and its acheived by the following peice of code.
CONNECT STRMADMIN/STRMADMINPW@<CONNECT_STRING_SOURCE>
DECLARE
V_SCN NUMBER;
BEGIN
V_SCN := DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER();
DBMS_APPLY_ADM.SET_TABLE_INSTANTIATION_SCN@DB_LINK_TARGET_DB(
SOURCE_OBJECT_NAME => '<SCOTT.EMP>',
SOURCE_DATABASE_NAME => 'SOURCE_DATABASE',
INSTANTIATION_SCN => V_SCN);
END;
STRMADMIN : Is a genenice user account (streams administrator) to manage oracle streams.

SCN and undo

hi,
say we start a transaction, and then we want to read a value, and that value has not been changed since our transaction started, but it is in a data block where another row in it has been changed since our transaction started.
So Oracle will see that that block's SCN is later than our transactions, and so it will go to the undo segment. This makes no sense to me.
Please help.
thanks

OracleGuy777 wrote:
hi,
say we start a transaction, and then we want to read a value, and that value has not been changed since our transaction started,This means that the block's transaction header has not recorded any active transaction and there is no Transaction SCN assigned within it. Whatever is there in the block, it can be read as it is without any kind of modifications done over it.
but it is in a data block where another row in it has been changed since our transaction started. That value change should not effect you. I believe what you are trying to say that there in the block, you got two values, Aman and employee code 1234 and you want to read the value Aman which is unchanged. The second value is getting updated ,may be to 5678 right?
So Oracle will see that that block's SCN is later than our transactions, and so it will go to the undo segment. This makes no sense to me.Okay, this certainly makes no sense. If you are not querying what's not changed, there is no point for Oracle to read the data from Undo for it. Oracle would do a current read for the data and would give the same content to you.
>
Please help.HTH
Aman....
thanks

Scn and seq number

is there any difference between the scn number and the log sequence number.please say me
Regards
Aram

do NOT cross/multi-post
scn number

Pixel Aspect and Image Squence

Hi, having a problem with pixel aspect ratios (PAR) . I created an 1024 x 768 image sequence in Maya with a PAR of 1.0. Opened in Photoshop- all is fine. Opened in Motion with same settings (height and width and a PAR of 1.0) and the image is stretched or cut off. Went to the media tab and tired all setting with the PAR and no dice. Used custom PAR and "from file" settings and still no dice. Went to Final Cut, imported, set h&W in the sequence setting with same dimension and PAR of 1.0. Much closer to source image but cropped a little at the top and bottom. I tried this with several files with differing h & w dimensions and all are problematic in Motion except if the h & w dimension ratio is 1.0 (e.g., 1000 X 1000 with a PAR of 1.0). Any help? Thanks.

I think I'm getting closer to narrowing down the problem:
1: If I render an image in Maya with an alpha channel and import into Motion, the aspect ratio in the Canvas is not displayed correctly (but is displayed correctly in the Utility window).
2: If I render an image without an alpha channel the aspect ratio in the Canvas is not displayed correctly (but is displayed correctly in the Utility window).
3: If I render an image in Maya with an alpha channel then remove the alpha channel in Photoshop then import into Motion, the aspect ratio in the Canvas is displayed correctly and all is good though this is not an acceptable workaround with a large number of images.
How do you post an image?

Last SCN in RMAN backups

Hi,
in 10g R2 how to know Last SCN in RMAN backups (to which we can recover) ?
Thank you.

Hi,
try to use the LIST command acorss backupsets or data files as per your need / requirement. Checkfor column
Ckp SCN and time Ckp Time. Based on that you find which is lastest, same works for Archive log too, perhaps check with Next SCN
- Pavan Kumar N
- ORACLE OCP - 9i/10g
https://www.oracleinternals.blogspot.com

Header datafile and SCN

Hi every body,
The controlfile contains the current SCN and the checkpoint SCN. that is right ?
In my search, I found that every datafile online and in read write mode, contain in his header the scn checkpoint.
But, I think that it contains only the last scn applied to this datafile. that is right?
Without this last scn in the datafile, how oracle know when he is stopped writing to the datafile in case of failure, to recover it ?
Thank in advance for your answers.

user622061 wrote:
Thank you a lot for the links.
I did a test :
BEFORE TEST
v$database :
CHECKPOINT_CHANGE# CURRENT_SCN
13114719 13117108
v$datafile :
NAME CHECKPOINT_CHANGE# LAST_CHANGE#
C:\ORACLE\ORADATA\BDD10G\TBS1_01.DBF 13117132
v$datafile_header :
NAME CHECKPOINT_CHANGE# LAST_CHANGE#
C:\ORACLE\ORADATA\BDD10G\TBS1_01.DBF 13117132
v$logfile and v$log :
MEMBER STATUS
C:\ORACLE\ORADATA\BDD10G\REDO01.LOG INACTIVE
C:\ORACLE\ORADATA\BDD10G\REDO02.LOG CURRENT
C:\ORACLE\ORADATA\BDD10G\REDO05.LOG UNUSED
TEST
create table toto ( a varchar2(30)) tablespace tbs1 ;
insert into table toto values ('SEBASTIAN');
commit;
AFTER TEST
v$datafile :
NAME CHECKPOINT_CHANGE# LAST_CHANGE#
C:\ORACLE\ORADATA\BDD10G\TBS1_01.DBF 13117132
v$datafile_header :
NAME CHECKPOINT_CHANGE# LAST_CHANGE#
C:\ORACLE\ORADATA\BDD10G\TBS1_01.DBF 13117132
the checkpoint_change# for this datafile is the same, before and after insert and commit a data in this tablespace.That's correct! The reason for this is that Oracle won't go and ask DBWR to write into the datafile with each and every commit of yours. You have ended a transaction, that's all! Now, the results of that transaction is updated already by LGWR into the redo log file. The checkpoint number(not SCN) is not updated by DBWR since this would happen in the case it would do a full Checkpoint.
To inspect the contents of this datafile, I use some OS commands : I search the word "SEBASTIEN" !
The first time, I didn't find it.
few minutes later, I found the word "SEBASTIEN" --> So my datafile is updated by the insert I did.Correct again! DBWR would write when it would think it's good to write. So not at the time of commit but after some time, DBWR updated the contents into the datafile.
But, the problem is that his checkpoint_change# still the same = 13117132As I have explained , it won't change, not so quickly.
>
>
You can reproduce the test !
My problem is to understand how Oracle ( DBWR) can update a datafile with data related to the SCN newer than the SCN registered in the header of datafile ?DBWR would have the list of the dirty buffer's list with it which it would periodically. If there is a commit and the data is updated in the logfile, the contents logged into the redo log file would be checkpointed in thedatafile after some time. This commit event, stored in the log file, is also recorded inthe control file. That's how Oracle knows, in the event of a crash, that to what extent they have to do the recovery since the stop checkpoint is already noted in the controlfile.
If Oracle did it, how can he know the last SCN on every datafile.It doesn't need to. Control file has it.
HTH
Aman....

Restoring backups older than last one.

hi,
If we have to restore rman backup older then the last backup, do we have to use TAG to restore it or is there any other method also available by which we can tell catalog to restore backup older then last backup.
Kindly assume oracle 9i version is there.

If you have sufficient backup available for the day of request, then you can use until time or until scn, and RMAN is intellegent enough to find the required backup closer to the date and apply required archive logs on top of it
Regards,
Ganesh

How to find the timestamp and SCN in the standby database?

Hai,
I have Oracle 9.2.0.4 RAC with 2 nodes in the production. The logs generated at these servers will be manully moved to my standby database and will be applied. To know what isthe maximum log files applied in the standby database, i am using the below mentioned query in the standby database,
Select thread#,max(sequence#) from v$log_history group by thread#
In general i am using "recover standby database until cancel" command and then checking the database with the above mentioned query whether all the logs are applied or not.
If i use time based or scn based recovery in standby database i.e., "recover standby database until time <time>" or "recover standby database until change <scn number>" , after completion of the recovery, apart from the message "Media recovery complete" or by seeing the alert log, is there any way to query the standby database, so that i can identify the time or scn upto which the archived redo log files got applied.

Hi Sridhar,
There should be some view which will have applied_scn information. There is one more option i can suggest, you can create a heart beat table in production with 2 column like scn and timestamp. Update this table every minute. From standby db you can query this table and get fair idea on applied_scn and timestamp.
While exporting you can export using flashback_scn by taking the value from heartbeat table of standby.
This heartbeat table is used very common in streams environment. Just see if this helps you.
hth,
http://borndba.com

Data Concurrency and Consistency ( SCN , DATA block)

Hi guys, i am getting very very very confused about how oracle implement consistency / multiversioning with regards to SCN in a data block and transaction list in the data block..
I will list out what i know so you guys can gauge me on where i am..
When a SELECT statement is issued, SCN for the select query is determined. Then Blocks with higher SCN are rebuilt from the RBS.
Q1) The SCN in the block implied here - is it different from the SCNs in the transaction list of the block ? where is this SCN store ? where is the transaction list store ? how is the SCN of the block related with the SCNs in the transaction list of the block ?
Q2) can someone tell me what happen to the BLOCK SCN and the transaction list
of the BLOCK when a transaction start to update to a row in the block occurs.
Q3) If the BLOCK SCN reflects the latest change made to the block and If the SCN of the block is higher then the SCN of the SELECT query, it means that the block has change since the start of the SELECT query, but it DOESNT mean that the row (data) that the SELECT query requires has changed.
Therefore why cant ORACLE just check to see whether the row has changed and if it has, rebuilt a block from the RBS ?
Q4) when ORACLE compares the BLOCK SCN, does it only SCAN for the BLOCK SCN or does it also SEARCH through the TRANSACTION LIST ? or it does both ? and why ?
Q5) is transaction SCN same as Transaction ID ? which is store in the RBS , the transaction SCN or ID ?
Q6) in short i am confuse with the relationship between BLOCK SCN, transaction list SCN, their location, their usage and relationship of the BLOCK SCN and transaction list when doing a SELECT, their link with RBS..
any gurus clear to give me a clearer view of what is actually happening ?

Hi Aman
Hmm agreed.So when commit is issued , what happens at that time?Simply put:
- The SCN for the transaction is determined.
- The transaction is marked as committed in the undo header (the commit SCN is also stored in the undo header).
- If fast cleanout takes place, the commit SCN is also stored in the ITL. If not, the ITL (i.e. the modified data blocks) are not modified.
So at commit, Oracle will replace the begin scn in the ITL with this scn
and this will tell that the block is finally committed is it?The ITL does not contain the begin SCN. The undo header (specifically the transaction table) contains it.
I lost here.In the ITL , the scn is transaction SCN or commit scn?As I just wrote, the ITL contains (if the cleanout occured) the commit SCN.
This sounds like high RBA information?What is RBA?
Commit SCNThis is the SCN associated with a committed transaction.
Begin SCNThis is the SCN at which a transaction started.
Transaction SCNAs I wrote, IMO, this is the same as the commit SCN.
Also please explain that what exactly the ITL stores?If you print an ITL slot, you see the following information:
BBED> print ktbbhitl[0]
struct ktbbhitl[0], 24 bytes     @44
      struct ktbitxid, 8 bytes    @44
         ub2 kxidusn              @44       0x0009
         ub2 kxidslt              @46       0x002e
         ub4 kxidsqn              @48       0x0000fe77
      struct ktbituba, 8 bytes    @52
         ub4 kubadba              @52       0x00800249
         ub2 kubaseq              @56       0x3ed6
         ub1 kubarec              @58       0x4e
      ub2 ktbitflg                @60       0x2045 (KTBFUPB)
      union _ktbitun, 2 bytes     @62
         b2 _ktbitfsc             @62       0
         ub2 _ktbitwrp            @62       0x0000
      ub4 ktbitbas                @64       0x06f4c2a3- ktbitxid --> XID, the transaction holding the ITL slot
- ktbituba --> UBA, used to locate the undo information
- ktbitflg --> flags (active, committed, cleaned out, ...)
- _ktbitfsc --> free space generated by this transaction in this block
- _ktbitwrp+ktbitbas --> commit SCN
HTH
Chris

How to logon to the Career Center using you SCN username and password

We all know how painful it is to have to create yet another username and password for a site, well for the Career Center you don't have to. We are working to have single sign on between SCN and the Career Center but until that time or if you navigate directly to the Career Center site you can log on using your SCN username and password via OpenID. Here is how.
First, ensure you have set an OpenID username in your SCN profile.
Then go to the Career Center http://scncareercenter.jobtarget.com and click
"Access Your Job Seeker Account" for job seekers OR
"Access Your Employer Account" for employers
At the bottom of the login box, besides "Have an OpenID?" click on the "Sign In" link
In the box type
http://openid.sap.com/your_open_id_name_from_your_SCN_profile
and click Login.
You will be taken to another page on SCN requesting you to login (if you haven't already) and finally approve the JobTarget OpenId login request.
Viola, you are then brought to your personal account on the Career Center. If this is your first time use, you may need to fill out some necessary pieces for the Career Center account profile.
Update 24.08.09: We are having some intermittent OpenID issues, will update the post when it is resolved
Cheers
Kuhan

That's really good news!
Now we have additional link to suggest newbie or job seeker in ABAP
P.S. Can you please amend in Forum Rules of Engagement with this information to let the others know?
Cheers

1Z0-052. Two big cert guides and official documentation - why so many contradictions ?

In this discussion i will compare two 1Z0-052 prepare certification guides (one of these is marked as "Oracle Press" !!!) and official Oracle Database 11g Release 2 (11.2) documentation (the exam "1Z0-052" has been validated against this documentation) in effort to understand why these sources have so many contradictions (direct and undirect).
Begin with this one :
1) "(Oracle Press) John Watson - OCA Oracle Database 11g: Administration I Exam Guide (Exam 1Z0-052)", "Chapter 2: Exploring the Database Architecture", page 54 :
"Note that from release 8i onward, checkpoints do NOT occur on log switch..."
2) "Oracle Database Concepts 11g Release 2 (11.2) Part Number E25789-01", "13 Oracle Database Instance", "When Oracle Database Initiates Checkpoints" :
Thread checkpoints OCCUR in the following situations:
- Consistent database shutdown
- ALTER SYSTEM CHECKPOINT statement
- Online redo log switch
3) "Biju Thomas - Oracle Database 11g Administrator Certified Associate study guide", "Chapter 8 Introducing Oracle Database 11g Components and Architecture", page 413 :
"Checkpoints OCCUR automatically when an online redo log file is full (a log switch happens)."
My question is :
Does checkpoint occur or not on log switch ?
Many other contradiction follows....

Hi John,
it will be my sixth certification in the last 3 years (i have all major certifications in Java)... But it is the first time i see that the Official Documentation of the product doesn't reflect completely the Certification Preparation Guide (i'm not going to say that you book is wrong, i'm about the complete compliance).
I cannot place my questions in another oracle DBA forum, because there most frequently the responses are "If you see the Official Documentation you find that..." and now i know that Official Documentation sometimes mismatch for the exam.
You (and I) certainly know that the certification exam is the multiple-choice questions... If i will find the question :
When checkpoints occurs (select all valid questions) ?
1) Consistent database shutdown
2) Issue of "ALTER SYSTEM CHECKPOINT" statement
5) Online redo log switch
after your explanation (see your first reply) now i know what can i must check and what i must not check, but before your explanation with small knowledges of database how can i be shure that your information is correct if the Official Documentation says the contrary ?
If i will find the question :
What is the checkpoint position ?
1) is the SCN (the System Change Number) in the redo stream where instance recovery must begin
2) ...
3) ...
If read only your book i cannot see SCN, i see RBA. How can i know that SCN and RBA are the same think if neither Oracle Documentation neither your book say that ? What another sources or guides i must have ?
And what about the location of the "alert_SID.log" file ?
What you suggest me about this exam question :
Which initialization parameter determines the location of the alert log file?
1) DIAGNOSTIC_DEST
2) BACKGROUND_DUMP_DEST
3) ALERT_LOG_DEST
4) USER_DUMP_DEST
If i read you book i select "BACKGROUND_DUMP_DEST", because in your book i see :
"The alert log is a continuous record of critical operations applied to the instance and the database. Its location is determined by the instance parameter BACKGROUND_DUMP_DEST, and its name is alert_SID.log, where SID is the name of the instance.... The directory ADR_HOME/trace (ADR is automatic diagnostic repository) is used as the default value for the instance parameters USER_DUMP_DEST (trace files generated by user sessions) and BACKGROUND_DUMP_DEST (the alert log and trace files generated by background processes)."
But if i read Official Oracle documentation i select "DIAGNOSTIC_DEST", because i see :
"The parameter BACKGROUND_DUMP_DEST is ignored by the new diagnosability infrastructure introduced in Oracle Database 11g Release 1 (11.1), which places trace and core files in a location controlled by the DIAGNOSTIC_DEST initialization parameter."
Therefore if i read you book i select "B", if i read documentation i select "A".
You must understand me - the only my goal is to be maximum precise - this precision is required for exam. I also have not free time to waste - during the week i'm working, i'm preparing for this OCA DBA certification, during weekend i'm writing my book in mathematics for preschool childs (it's also about 700 pages)...
Excuse me and thank you,
Dmitri.

RMAN level 0 and level 1 tablespace/database relationships

If I take an incremental level 0 backup of an entire database and then take a level 0 of an individual tablespace, does an incremental level 1 backup of that tablespace reference the incremental level 0 of the tablespace or of the entire database? If I change the order of the level 0 backups, does it change the reference of the tablespace's level 1?
I guess what I'm asking is does the the level 1 incremental reference the latest level 0 regardless of whether it's for the whole database or just the tablespace or does a level 1 always reference it's own level 0?
Also, if I take a level 0 of the database Sunday night, and then a level 1 of the database every night after that as well as multiple level 1s of a tablespace throughout each day, will Tuesday night's database level 1 include all of Monday's tablespace level 1s?
I just can't seem to find oracle documentation about these relationships.

Hemant K Chitale wrote:
"Database" and "Tablespace" are logical groupings for our convenience.
RMAN tracks backups at the datafile level.I think that is exactly what I was looking for. I just want to clarify my understanding.
If I run a level 0 on the database Sunday night, a database differential level 1 each night, and a level 0 on a specific tablespace each morning at 6AM followed by hourly tabelspace differential incrementals only for that tablespace, then Monday night's level 1 will be an incremental of the entire database pointing to Sunday's level 0 except for that specific tablespace for which it will be incremental from that tablespace's last level 1. Does that sound right? If so, then it sounds like to restore the entire database to its state on Tuesday at noon from media, I have to put these backups back on disk before running the RMAN restore:
- database level 0 from Sunday
- Monday night's database level 1
- Tuesday's tablespace level 0
- Tuesday's tablespace level1s from 7AM - noon
This means I can avoid putting Monday's tabelspace incrementals back right? From what I gather, the RMAN restore will put the blocks back into the datafiles at their correct SCNs, and then if there are any archive logs after that point in time, the RMAN recover will apply them. Does that all sound correct?

SCN_TO_TIMESTAMP and TIMESTAMP_TO_SCN Behaviour

Dear all,
I'm trying to understand how both SCN_TO_TIMESTAMP and TIMESTAMP_TO_SCN work by executing the following:
SQL> select scn_to_timestamp(ora_rowscn) test, last_name from employees where la
st_name='King';
TEST                 LAST_NAME
16-DEC-10 01.34.23.0 King
00000000 PM
16-DEC-10 01.34.23.0 King
00000000 PM
SQL> select ora_rowscn, last_name from employees where last_name='King';
ORA_ROWSCN LAST_NAME
    559374 King
    559374 King
SQL> select timestamp_to_scn('16-DEC-10 01.34.23.000000000 PM') from dual;
TIMESTAMP_TO_SCN('16-DEC-1001.34.23.000000000PM')
                                           559370why doesn't the last query resulting 559374 but instead 559370?
best regards,
valerie

Hi,
From the documentation :
SCN_TO_TIMESTAMP takes as an argument a number that evaluates to a system
change number (SCN), and returns the approximate timestamp associated with that
SCN.The important word being approximate.
You cannot rely on it to be as precise as a systimestamp.
timestamp_to_scn(scn_to_timestamp(SCN)) == SCN is FALSE
(even if it might happen to be true sometime...)

Scn in recovery

Hi
1-)
If I restored a datafile which has higher scn then controlfile and datafiles.
I think The only way to open the db is to recreate a controlfile or copy the old controlfile.
Correct me if I am wrong.
2-)
If I recreate the controlfile how does oracle determine checkpoint scn of the datafiles?
3-)
Even I`ve just recreated a new controlfile and copy a datafile which has high scn then other datafiles.
When I check;
select CHECKPOINT_CHANGE# from v$datafile, all numbers are equal.
What is the reason for that?

Situation 1) would arise when you have backups of datafiles from different points in time and you restore almost all the datafiles from a set prior to the last backup but one datafile from the last backup. (you can't be restoring a datafile from the future ! so you can't have restore a datafile with a higher SCN than the current SCN, ergo, the other datafiles have also been restored from a backup older than this one !)
In this case,
a) if you restore the controlfile, you use the "USING BACKUP CONTROLFILE" clause.
b) if you recreate the controlfile, you use the "USING BACKUP CONTROLFILE" clause.
(ie, the RECOVERY command is the same !)
2) : Oracle reads the datafile headers. It uses this information to determine the lowest SCN in the datafiles and identify which Log Sequence Numbers are required for recovery. Don't think of whether "oracle uses datafile headers to create the controlfile" {although it DOES do so}. Think of "oracle uses the datafile headers to determine SCNs and LSNs for Recovery !".
3) : Checkpoint Change# is as of the last Checkpoint. A Checkpoint could be a system checkpoint ("ALTER SYSTEM CHECKPOINT") or a Checkpoint initiated by a Log Switch or an Incremental Checkpoint (because of FAST_START_MTTR_TARGET) or a Tablespace Checkpoint (ALTER TABLESPACE command) or a Datafile checkpointed (ALTER DATABASE DATAFILE command). (Note : You will not see Incremental Checkpoints in the V$DATAFILE_HEADER)
Here's one example below. Notice one datafile having a different CHECKPOINT_CHANGE# with the ALTER TABLESPACE command.
SQL> select checkpoint_change# from v$datafile_header;
CHECKPOINT_CHANGE#
           1183007
           1183007
           1183007
           1183007
           1183007
           1183007
6 rows selected.
SQL> alter tablespace users begin backup;
Tablespace altered.
SQL> select checkpoint_change# from v$datafile_header;
CHECKPOINT_CHANGE#
           1183007
           1183007
           1183007
           1204726
           1183007
           1183007
6 rows selected.
SQL> alter tablespace users end backup;
Tablespace altered.
SQL> select checkpoint_change# from v$datafile_header;
CHECKPOINT_CHANGE#
           1183007
           1183007
           1183007
           1204726
           1183007
           1183007
6 rows selected.
SQL> Hemant K Chitale
http://hemantoracledba.blogspot.com
Edited by: Hemant K Chitale on May 31, 2009 11:41 PM

Last SCN and last squence

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