Explain how to increase or decrease the parameter compatible in Oracle

I am reading the Oracle 10 Gb documentation today and have noticed irreversible compatibility.

Starting with Oracle Database 10g, the compatible initialization parameter becomes irreversible; that is, it cannot be set to a value that is less than a previous value. therefore, the compatibility of the database can go only forward and never backward.
For example, suppose that you set compatible to 10.0.0 and start up the database, then shut down the database to restart with compatible set to 9.2.0. during startup, you get an error indicating that the compatible setting cannot be reversed.
If you do advance the compatibility of your database with the compatible initialization parameter, there is no way to start the database using a lower compatibility level setting, cannot t by doing a point-in-time recovery to a time before compatibility was advanced.
Any changes since, are lost.

Consequently, the alter database reset compatibility command is now obsolete.

Since 10 Gb, compatible cannot be reduced from high. Can it be 9i? Change compatiable from 9.2.0.5 to 9.2.0.1? I can test it.

Previously, I thought that compatible could not be changed to a smaller value in all versions (including Patch Set), and thought that every modification to compatiable would modify all datafile headers.

Let's take a look at the 10 Gbit/s command. After downgrading compatible, the database is incorrect when it is mounted. The parameter settings are inconsistent with the control file.

SQL> startup
Oracle instance started.

Total system global area 272629760 bytes
Fixed size 2035592 bytes
Variable Size 201330808 bytes
Database buffers 62914560 bytes
Redo buffers 6348800 bytes
ORA-00201: control file version 10.2.0.3.0 incompatible with Oracle version 10.2.0.1.0
ORA-00202: Control File: '/Oracle/DDS/data03/lewu/data/cntrl_lewu_1.dbf'

Modify compatiable from 9.2.0.5 to 9.2.0.0.0. The database is restarted successfully. View logs.

Starting Oracle instance (normal)
Starting up Oracle RDBMS version: 9.2.0.5.0.
System Parameters with non-default values:
Compatible = 9.2.0.5

Arch: Starting arch Processes
Arch: Starting arch processes complete
Alter database Mount
Completed: Alter database Mount
Wed Aug 27 20:53:23 2008
Alter database open
Completed: Alter database open

Alter database close normal
Completed: Alter database close normal

Starting Oracle instance (normal)
Starting up Oracle RDBMS version: 9.2.0.5.0.
System Parameters with non-default values:

Compatible = 9.2.0.0

Arch: Starting arch Processes
Arch: Starting arch processes complete
Alter database Mount
Completed: Alter database Mount
Alter database open
Completed: Alter database open

Then, let's see if we can change from 9.2.0.5 to 8.1.7.4. The result fails. The database can be mounted but cannot be started. Cannot see cross-major version.

SQL> startup
Oracle instance started.

Total system global area 689409008 bytes
Fixed size 732144 bytes
Variable Size 352321536 bytes
Database buffers 335544320 bytes
Redo buffers 811008 bytes
Database mounted.
ORA-00402: Database changes by release 9.2.0.0.0 cannot be used by release 8.1.7.0.0
ORA-00405: Compatibility type "locally managed system tablespace"

Oracle @:/export/home/Oracle/products/9205/DBS> oerr ora 00402
00402,000 00, "database changes by release % s cannot be used by release % s"
// * Cause: Changes have been made to the database that require a newer
// Software release or that violate the compatibility parameters.
// * Action: Use a version of the software that can understand the changes or
// Relax the compatibility requirements in the init file.

Conclusion Correct the previous misunderstanding: compatible can decrease or increase at 9i. 10g cannot begin.

 

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