ORA-28113: policy predicate has error - database

Question

I need some help with Oracle's VPD feature. I have never used it before but did some research online about it, however I'm running into a problem.
Here are the steps that I have taken:
QuanTriDL:
create table NhanVien2
table NhanVien2
QuanTriVPD:
CREATE OR REPLACE CONTEXT ThongTinTaiKhoan USING TTTK_PKG;
CREATE OR REPLACE PACKAGE TTTK_PKG IS
PROCEDURE GetTTTK;
END;
/
CREATE OR REPLACE PACKAGE BODY TTTK_PKG IS
PROCEDURE GetTTTK AS
TaiKhoan varchar(30);
tenPhong varchar(30);
tenChucVu varchar(30);
tenMaNV varchar(10);
BEGIN
TaiKhoan := LOWER(SYS_CONTEXT('USERENV','SESSION_USER'));
DBMS_SESSION.set_context('ThongTinTaiKhoan','GetTaiKhoan',TaiKhoan);
if (TaiKhoan = 'nv001') then
DBMS_SESSION.set_context('ThongTinTaiKhoan','GetChucVu','Giam doc');
else
if (TaiKhoan = 'nv002') then
DBMS_SESSION.set_context('ThongTinTaiKhoan','GetChucVu','Truong phong');
DBMS_SESSION.set_context('ThongTinTaiKhoan','GetPhong','Kinh doanh');
else
if (TaiKhoan = 'nv006') then
DBMS_SESSION.set_context('ThongTinTaiKhoan','GetChucVu','Truong phong');
DBMS_SESSION.set_context('ThongTinTaiKhoan','GetPhong','Ky thuat');
else
DBMS_SESSION.set_context('ThongTinTaiKhoan','GetChucVu','Nhan vien');
end if;
end if;
end if;
EXCEPTION
WHEN NO_DATA_FOUND THEN NULL;
END GetTTTK;
END;
/
CREATE OR REPLACE TRIGGER RangBuocTTTK AFTER LOGON ON DATABASE
BEGIN QuanTriVPD.TTTK_PKG.GetTTTK;
EXCEPTION WHEN NO_DATA_FOUND
THEN NULL;
END;
/
then:
CREATE OR REPLACE FUNCTION Select_Nhanvien(
schema_p IN VARCHAR2,
table_p IN VARCHAR2)
RETURN VARCHAR2
AS
getChucVu varchar(50);
trave varchar2(1000);
BEGIN
SELECT SYS_CONTEXT('ThongTinTaiKhoan','GetChucVu') into getChucVu FROM DUAL;
trave := '1=2';
if (getChucVu = 'Giam doc') then
trave := NULL;
else
if (getChucVu = 'Truong phong') then
trave :='Phong=(SELECT SYS_CONTEXT(''ThongTinTaiKhoan'',''GetPhong'') FROM DUAL)';
else
trave :='TenTaiKhoan=(SELECT SYS_CONTEXT(''ThongTinTaiKhoan'',''GetTaiKhoan'') FROM DUAL)';
end if;
end if;
RETURN trave;
END;
/
BEGIN
DBMS_RLS.ADD_POLICY (
object_schema => 'QuanTriDL',
object_name => 'NhanVien2',
policy_name => 'VPD_Select_Nhanvien',
function_schema => 'QuanTriVPD',
policy_function => 'Select_Nhanvien',
statement_types => 'SELECT');
END;
/
When connecting as nv001, nv002, nv006 it's OK. But connecting another user:
ORA-28113: policy predicate has error
Why does it cause that error?

Answer 1

(year old question but since I stumbled across it I'll go ahead and answer it for anyone else...)
ORA-28113 just means that when your policy function returned a where clause, the resulting SQL had some error. You can get details by looking at the trace file. Also, try:
select Select_Nhanvien('myschema','mytable') from dual;
And then append the results to a WHERE clause like this:
SELECT * FROM MYTABLE WHERE <results from above>;
And then you should see the root cause. I'm guessing in the case above the 'other user' didn't have either the sys_context variables required to build the where clause, or access to the login trigger.
As a side note, another problem you can run into here is circular reference when your policy function references its own table - ideally I would expect a policy function to bypass itself within the policy function so you can do NOT EXISTS, etc but it doesn't seem to work that way.