In Oracle 11g if global index maintained during execution of the DROP partition command then command could take hours to complete. Oracle 12c said DROP partition command executes immediately and index will remain usable.

I had database table in Oracle 11g which use partition drop for purging. But never able to utilize partition drop due to above 11g index unusable issue. So I updated the database to Oracle 12c. But still when I drop the partition index become unusable. I disabled the Oracle database backward compatibility but still issue is there. Interestingly when I created a new table in 12c it works as expected. But not for tables upgraded from 11g to 12c.