what makes oracle not to allow to downgrade

[Lakshmi N]

Hi,

What makes oracle not to allow to down grade after changing the compatible parameter.

What is the link like if we set lower version we cannot use features,if set cannot downgrade so upgrading to higher versions needs change in compatible change.

Regards
Lakshmi N

[ErmanArslansOracleBlog]

Hi,

using a higher version compatible parameter makes Oracle to use the new features. this new features may store its own data on disk, so it can change the structures in the disk.. So if you downgrade in such a scenario, the downgraded database may not know the new shape of the disk structure.. This is the basic answer for your question and that's why Oracle prevents such downgrades..
For instance,  if you upgrade to 10g and set the compatible to be 10.2.0 then you can not downgrade directly to 9i.. In such situation you need to restore a backup.