How to create session variables for UserID in obiee?

http://forums.oracle.com/forums/thread.jspa?messageID=3447246&#3447246
http://forums.oracle.com/forums/thread.jspa?messageID=3376727&#3376727
http://gerardnico.com/wiki/dat/obiee/saw_url_session_variable

fetching dashboards login userid
session variable:
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DISPLAYNAME
init block:
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datasource type is OBIEE server
SELECT username,groupname from usera where username=':USER'