下面是Oracle 19 c Database PL/SQL Language Reference文档PL/SQL Subprograms 8.14（着重号是后加的）的摘录。
“在服务器调用期间，当DR [DEFINER RIGHT]单元被推送到调用堆栈时，数据库存储当前启用的角色以及CURRENT_USER和CURRENT_SCHEMA的当前值。然后，它将CURRENT_USER和CURRENT_SCHEMA都更改为DR单元的所有者，并仅启用***PUBLIC角色。”

select * 
  from dba_sys_privs 
 where     grantee   in ('A', 'PUBLIC') 
       and privilege  = 'ALTER SESSION'
/
-- no rows returned

PUBLIC未被授予ALTER SESSION系统权限。
用户A没有直接被授予ALTER SESSION系统权限，而是通过角色被授予ALTER SESSION。

create or replace procedure alter_session
  as
  begin
    execute immediate 'alter session force parallel ddl';
  end;
/

SQL*Plus: Release 19.0.0.0.0 - Production on Sat May 13 20:09:02 2023
Version 19.3.0.0.0

Copyright (c) 1982, 2019, Oracle.  All rights reserved.

Last Successful login time: Sat May 13 2023 20:08:48 -04:00

Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.19.0.0.0

SQL> alter session enable parallel ddl
  2  /

Session altered.

SQL> select pddl_status from v$session where username = 'A'
  2  /

PDDL_STA
--------
ENABLED

SQL> begin
  2    alter_session();
  3  end;
  4  /

PL/SQL procedure successfully completed.

SQL> select pddl_status from v$session where username = 'A'
  2  /

PDDL_STA
--------
FORCED

即使用户A的角色没有被定义者正确的PL/SQL程序单元的执行（alter_session()）使用，用户A是如何通过调用alter_session()来改变她的会话的？