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, except 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.
从10g开始COMPATIBLE 不能由高降低，难道9I可以？将compatiable从9.2.0.5改为9.2.0.1？我测试后果然可以。
以前我一直以为COMPATIBLE 在所有版本(包括patch set)都不能修改为更小的值，以为每次修改compatiable都会去修改所有的datafile header.
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'
看日志.
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
数据库能够mount，但无法启动。看不来不能够跨大版本。
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, 00000, "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.
结论纠正以前的一个误区:compatible在9i的时候能降能升。10g开始不行了。