Comments on: sequence / thread / system change number

By: Vinod

Vinod — Fri, 26 Sep 2008 12:02:13 +0000

SCN is a differnet kind of sequence number. It is composed of two things
1. Timestamp 2. Sequence number. Since timestamp will always be unique the SCN will also be unique. Since timestamp portion is attached with it no need to worry that it will hit the maximum number.

By: Laurent Schneider

Laurent Schneider — Wed, 11 Jul 2007 14:48:48 +0000

a big note about timestamp_to_scn… it is not at all accurate


SQL> select current_timestamp,
  2    timestamp_to_scn(current_timestamp) timestamp_to_scn,
  3    current_scn
  4  from v$database;

CURRENT_TIMESTAMP              TIMESTAMP_TO_SCN CURRENT_SCN
------------------------------ ---------------- -----------
16:48:11.477439                        10017141    10017179

SQL> select current_timestamp,
  2    timestamp_to_scn(current_timestamp) timestamp_to_scn,
  3    current_scn
  4  from v$database;

CURRENT_TIMESTAMP              TIMESTAMP_TO_SCN CURRENT_SCN
------------------------------ ---------------- -----------
16:48:11.497542                        10017141    10017180

SQL> select current_timestamp,
  2    timestamp_to_scn(current_timestamp) timestamp_to_scn,
  3    current_scn
  4  from v$database;

CURRENT_TIMESTAMP              TIMESTAMP_TO_SCN CURRENT_SCN
------------------------------ ---------------- -----------
16:48:11.537012                        10017141    10017181

SQL> select current_timestamp,
  2    timestamp_to_scn(current_timestamp) timestamp_to_scn,
  3    current_scn
  4  from v$database;

CURRENT_TIMESTAMP              TIMESTAMP_TO_SCN CURRENT_SCN
------------------------------ ---------------- -----------
16:48:11.571872                        10017141    10017182

By: Laurent Schneider

Laurent Schneider — Tue, 26 Jun 2007 15:04:26 +0000

> 4 bytes

you mean SCN cannot exceed 2^32 (4 billions) ? This is really small, a busy system can reach this in one day… it is hard to believe to have a non-unique scn, how would RMAN figure out what RESTORE UNTIL SCN means, if scn is not unique?

Where did you read it is 4bytes?

Well, the big systems mostly run 64bits today, but still I am worried about your comment

By: Edgar

Edgar — Thu, 21 Jun 2007 17:44:33 +0000

Yes, but Oracle is storing this value everywhere, in datafile, in control file, passes from one instance to another, and it is not a Oracle “number” datatype, it is only 4 bytes (at least on 32bit platforms), so I think there should be some kind of solutions, that Oracle is using when SCN gets big

By: Laurent Schneider

Laurent Schneider — Wed, 06 Jun 2007 15:48:09 +0000

no, I do not think so. Really big is multi-billions, which is incredibly small for Oracle

By: Edgar

Edgar — Wed, 06 Jun 2007 12:43:00 +0000

Hello,

You said that SCN is continously increasing, what happens when it gets really big? Does this number overflow and starts from 0?

Thank you.