What is the issue with default temporary tablespace during the upgrade process from Oracle version 12.2 to 19c?

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The issue with default temporary tablespace during the upgrade process from Oracle version 12.2 to 19c is that the default temporary tablespace changes from SYSTEM to a new tablespace called TEMP. If the upgrade process fails to create the TEMP tablespace or assign it as the new default temporary tablespace, it can cause issues with various database operations, such as creating temporary tables and performing sorts. It is recommended to manually create the TEMP tablespace and set it as the default temporary tablespace before starting the upgrade process.