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Short answer - No.
You can't convert between common and local user accounts.
Even if you could, where would the database store the account's "data and objects"?
You should not have any user/application data in the Container Database.
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When I am in this situation (i.e. a query very slow in an instance and fast in another) I check what differences I can find between the environments:
Compare EXPLAIN PLAN for your count query, and for that subquery, on the different environments, it might give an idea about where to check for differences
Is the data different accross environments ? A ...
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