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ORA-00023:

session references process private memory; cannot detach session
Cause: An attempt was made to detach the current session when it contains references to process private memory.
Action: A session may contain references to process memory (PGA) if it has an open network connection, a very large context area, or operating system privileges. To allow the detach, it may be necessary to close the session's database links and/or cursors. Detaching a session with operating system privileges is always disallowed.