deadlock when trigger fired ?

Hello,

 I want to test “pending stats” via disabling all hints in my PL/SQL application.

 I have developed this trigger :

 CREATE OR REPLACE TRIGGER LOGON_TRIGGER

AFTER LOGON ON TOTO.SCHEMA

BEGIN

-- To use Pending Stats

EXECUTE IMMEDIATE 'ALTER session SET optimizer_use_pending_statistics=true';

-- To disable hints

EXECUTE IMMEDIATE 'ALTER session set "_optimizer_ignore_hints"=TRUE';

END;

 When application is lunched, deadlock error is encountered: ORA-04020.

 An idea?

Thanks