When you migrate, you should be prepared to see some execution plan changing. That's not new. But here I'll show you a case where you have several bad execution plans because lot of histograms are missing. The version is the same. The system is the same. You've migrated with DataPump importing all statistics. You have the same automatic job to gather statistics with all default options. You have repeated the migration several times on a system where you constantly reproduce the load. Have done a lot of regression tests. Everything was ok.
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