session_cached_cursors

Hello list ,
Performance tuning docs say that : "To enable the caching of session cursors you must set the initialization parameter set session_cached_cursors. You can also enable it dynamically with alter session set"

But neither of the following works on 9.2.0.1.0 enterprise edition (win32)

alter system set session_cached_cursors=10 scope=memory;
alter system set session_cached_cursors=10 scope=spfile;
alter system set session_cached_cursors=10 scope=both;

However this works
alter session set session_cached_cursors=10 ;

Why ?

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