For example, let's say you set the COMPATIBLE parameter to 10.0.0 on the Initialization Parameters page in Enterprise Manager (EM), and start the database. If you then shut down the database and restart with the COMPATIBLE parameter set to 9.2.0, an error displays during startup informing you that the compatible setting cannot be reversed.
A problem arises if you set the COMPATIBLE parameter to a value higher than the existing one. In so doing, you advance the compatibility of your database, and you won't be able to start the database using the true, lower-compatibility level setting. The only way out of this is to perform a recovery of an older instance of the database, before the COMPATIBLE parameter was changed, but any other changes you may have made to the database will also be lost. For this reason, the ALTER DATABASE RESET COMPATIBILITY command no longer works in Oracle Database 10g.
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