What is SCN number in Oracle? Plz any one give me the explanation for SCN

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opbang

  • Sep 22nd, 2006
 

System Change Number (SCN)

The system change number (SCN) is an ever-increasing value that uniquely identifies

a committed version of the database. Every time a user commits a transaction,

Oracle records a new SCN.  You can obtain SCNs in a number of ways, for example,

from the alert log. You can then use the SCN as an identifier for purposes of

recovery. For example, you can perform an incomplete recovery of a database up to

SCN 1030.   Oracle uses SCNs in control files, datafile headers, and redo records. Every redo log

file has both a log sequence number and low and high SCN. The low SCN records

the lowest SCN recorded in the log file, while the high SCN records the highest SCN

in the log file.

Kiran

  • Sep 26th, 2006
 

SCN is System Change Number. It is an unique incremental number in the database (as your clock time). The SCN number is incremented every 3 seconds. This number is very useful while recovering the database or instance. All the datafile headers will have the same scn number when the instance is shutdown normally. You can get the current scn number from dbms_flashback.get_system_change_number or if you are using 10g your can query v$database

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senthilora

  • Sep 24th, 2007
 

SCN number gets incremented by Oracle server irrespective of the user COMMIT.

SCN number always gets incremented by Oracle server.  It is not time bounded.  Only the Check-Points getting triggered every 3 seconds to make the DB files in sync with the Control file to mark the recovery point.

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senthilora

  • Dec 19th, 2007
 

Only the LGWR will be triggered for every 3 seconds.  CKPT will be happening based on the recovery settings we have given for the database.  for ex. FAST_START_MTTR_TARGET, when ever Log switch happening, alter tablespace offline / readonly...

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SCN is System Change Number. Whenever a transaction or set of transaction is commit
then SCN is assigned to that transaction. Each of redo logfile have its starting
and
ending checkpoint it means all the transaction whose SCN is in this range is
in that redo log file. Current SCN is get saved in control file. Whenever the
database get fully syncronised then all the data files header get updated with
current SCN.


You can see current SCN by dbms_flashback.get_system_change_number.


Whenever the database get shutdown normally then header of all the data file gets this same
System Changed Number.

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dj_dj_dj

  • Jun 23rd, 2009
 

System Change Number (SCN) This is a sequential counter, identifying precisely a moment in the database. This is the most accurate way to identify a moment in time. You can query SELECT CURRENT_SCN FROM V$DATABASE; Is a pseudo column of any table that is not fixed or external. It represents the SCN of the most recent change to a given row, that is, the latest COMMIT operation for the row.

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neelapu

  • Jun 23rd, 2009
 

System Change Number (SCN) This is a sequential counter, identifying precisely a moment in the database.This is the most accurate way to identify a moment in time.
You can query
SELECT CURRENT_SCN FROM V$DATABASE;
or
SELECT DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER FROM DUAL, to get the current change number.

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raj

  • Sep 1st, 2011
 

SCN is the system change number. It is nothing but the clock of the database. when you create the database the SCN is generated and for every change in the database there another SCN generated with a number. It is directly proportional to the timeline of the database.

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kriss

  • Sep 27th, 2011
 

(SCN) > System change num. its in KB Size & for every 3 seconds oracle automatically update into the Control file

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Vikas

  • Oct 8th, 2011
 

SCN is System Change Number. It is an unique incremental number in the database (as your clock time). The SCN number is incremented every 3 seconds. This number is very useful while recovering the database or instance. All the datafile headers will have the same scn number when the instance is shutdown normally. You can get the current scn number from dbms_flashback.get_system_change_number or if you are using 10g your can query v$database

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Mujeebur Rahman

  • Oct 9th, 2011
 

Scn number is system change number,whenever switch the log file scn number will change

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balu

  • Feb 15th, 2012
 

SCN will generate at online Redos at every one second.

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Muthupandian.A

  • Feb 24th, 2012
 

SCN-SYSTEM Change number, it will generate automatically while u perform commit.

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Sriniv@s..

  • May 12th, 2012
 

SCN is system change number.it will generate every time when log switch occurred & user commits the transaction.Main thing is scn will generated with every transaction &(or) every one seconds.

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Muthupandian

  • Jul 7th, 2012
 

SCN is a number.it will be generated into redo buffer while performing every transaction.it will store into redolog files during commits.it helps for recovery.

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SANTHOSH

  • Aug 15th, 2012
 

SCN no is a number which is generated by the oracle, when we made any changes to the data that statements will be recorded in redo log files and every committed statement will be given this SCN no by oracle.

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venu

  • Sep 1st, 2012
 

SCN is generated by oracle when a transaction is successfully commited. This is referred to as SystemChangeNumber. We can think of it as a transaction id generated when we make some transaction in an ATM. The committed transaction is written to the redo logs.
During the Instance Crash Recovery, SMON background process checks the SCN in Control file, redolog files.
If these two SCNs donot match, it is difficult to bring up the db.

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