why we run datapatch:

The datapatch utility will run the necessary apply scripts to load the modified SQL files into the database. Entires will be added to both DBA_REGISTRY_HISTORY and DBA_REGISTRY_SQLPATCH views automatically.

DEBAM1> SELECT patch_id, status FROM dba_registry_sqlpatch;

  PATCH_ID STATUS

---------- ---------------

  22139226 SUCCESS

  21948354 SUCCESS

  20204035 SUCCESS

  22139226 SUCCESS

---------- ---------------

  30805558 END

  20204035 END

  31219939 END

  31113348 BEGIN

it will update like begin then end then sucess

Why we use open resetlogs?

Qn 1 :

A RESETLOGS is required in either of

a. Recovery using BACKUP CONTROLFILE

b. Incomplete Recovery

If you are using a Backup controlfile (whether from a Binary Backup or actually via a CREATE CONTROLFILE), the Controlfile is not current -- therefore it does not have the database SCN and LogSequenceNumbers. The RESETLOGS updates this information back to the controlfile (normally, a Recovery is the other way round with the controlfile's SCN, being the highest, driving the Recovery).

Also, a Resetlogs is required in both cases to ensure that the older Redo Logs (e.g. they might still be on disk) are not used -- the Resetlogs creates a new Incarnation of the database.

Qn 2:

There is no difference between an ALTER DATABASE OPEN; and an ALTER DATABASE OPEN NORESETLOGS;.

The NORESETLOGS is the default action in an OPEN if you do not specify RESETLOGS.

(Obviously, Oracle check to see if you have used a Backup Controlfile and/or done an Incomplete Recovery, in which case it does not allow you to OPEN without the RESETLOGS).