Flashback query after restart

 Hi,
I'm getting conflicting advice on whether flashback query can work across a database restart - i.e. can I query to an AS OF time prior to the restart?
This document states that it cannot: Doc ID 435998.1  ,  but the Ora flashback documents imply that it will work, as UNDO is preserved.
This page http://www.databasejournal.com/features/oracle/article.php/3823171/Oracles-Flashback-Query--Should-you-unset-inmemoryundo.htm demonstrates that it only works if _in_memory_undo=false.

I have done my own testing, and have seen it working, without needing to set _in_memory_undo (i.e. using default of true). We would rather not change the default, as this has a performance impact.