Question

I am trying to define three database tables – USERNAME, USER_SESSIONS, and TOOLBOX_DIRS_REGISTERED. I can define the first two of these without any problems, but the last one is causing me a bit of grief. Ideally I would like to define it as is shown in the code block below, however I believe that sub-queries cannot be used within check constraints?

The code block immediately below shows how I would like to define the table TOOLBOX_DIRS_REGISTERED. The start of the code block also contains some comments which attempt to explain my thoughts about certain constraints which the table definition uses.

-- Constraint : USERNAME_FK
-- ========================
--
-- USERNAME must contain a username which has been added to the table USERNAME.
--
-- Constraint : USER_SESSION_ID_FK
-- ===============================
--
-- USER_SESSION_ID must contain a user session ID which has been added to the table 
-- USER_SESSION.
--
-- Constraint : check_user_session_id
-- ==================================
--
-- In addition to the above constraint, USER_SESSION_ID must also belong to the 
-- username which is contained within USERNAME.

create table
TOOLBOX_DIRS_REGISTERED
(
 DIRNAME         varchar2(100) not null,
 USERNAME        varchar2(32)  not null,
 USER_SESSION_ID varchar2(32)  not null,
 AUTO_REGISTER   char          not null,
 constraint
   TOOLBOX_DIRS_REGISTERED_PK
   primary key (DIRNAME),
 constraint
   USERNAME_FK
   foreign key (USERNAME)
   references USERS(USERNAME),
 constraint
   USER_SESSION_ID_FK
   foreign key (USER_SESSION_ID)
   references USER_SESSIONS(USER_SESSION_ID),
 constraint
   check_user_session_id
   check
   (
    USERNAME in
    (
     select USERNAME from USER_SESSIONS
     where USER_SESSIONS(USER_SESSION_ID) = USER_SESSION_ID
    )
   )
);

Does anyone know of a way around this problem, i.e. the use of the sub-query in the definition of the check constraint ‘check_user_session_id’? I read on Stackoverflow that Materialized views can be used instead of sub-queries in this particular situation. The trouble is, if I use a Materialized view, then I would want to make sure it was up to date at the time the check constraint was being performed. So what I did was implement the Materialized view along with a Trigger which called dbms_mview.refresh on the Materialized view. This is all fine, except that Oracle complains that it cannot COMMIT in a Trigger. Urgh! The exact message it gives me is ;

ERROR at line 2:
ORA-04092: cannot COMMIT in a trigger
ORA-06512: at "SYS.DBMS_SNAPSHOT", line 2760
ORA-06512: at "SYS.DBMS_SNAPSHOT", line 2740
ORA-06512: at "SYSTEM.SIMULAB_MVIEW", line 19
ORA-06512: at "SYSTEM.TRIG_TOOLBOX_DIRS_REGISTERED", line 3
ORA-04088: error during execution of trigger 'SYSTEM.TRIG_TOOLBOX_DIRS_REGISTERED'

I’m assuming that Oracle is attempting to automatically perform a commit after it refreshes the Materialized view and that’s what it is complaining about?

My Trigger is defined as follows ;

create or replace trigger TRIG_TOOLBOX_DIRS_REGISTERED
  before insert or update on
    TOOLBOX_DIRS_REGISTERED
  begin

    -- Invoke the PL/SQL Package procedure simulab_mview.refresh_mview

    simulab_mview.refresh_mview;

  end;

while the PL/SQL Package simulab_mview is defined as follows ;

create or replace
package
simulab_mview
as

    procedure
    refresh_mview;

end;
/

create or replace package body
simulab_mview
as

    procedure
    refresh_mview
    as

    begin

        -- I have a strong suspicion that dbms_mview.refresh might cause a commit to 
        -- be executed. This would make sense, as the RDBMS would need to execute a 
        -- commit so that other clients could see the result of the refresh.

        dbms_mview.refresh('mat_view', 'C');

    end;

end;
/

Does anyone have any thoughts on this? Is there an easier way to do what I want to do, should I not be using Materialized views, should I not be using Triggers?

Any help or even thoughts on this would be immensely appreciated. If anyone is indeed able to provide any help or thoughts on this matter, then I would like to thank them in advance for their assistance.

Have a nice day.

Answer 1

If I am understanding this correctly, I don’t think you want toolbox_dirs_registered to refer to both the users and the user_sessions tables. That isn’t normalized. Only refer to the user_sessions table and you already have a reference to the users table.

The following snippet is untested and has a lot of cut and paste, but I think it gives the idea:

CREATE TABLE users(
   user_id  NUMBER
  ,username VARCHAR2(32)
  ,CONSTRAINT users_pk PRIMARY KEY(user_id)
);

CREATE UNIQUE INDEX users_username_uk ON users(username);


CREATE TABLE user_sessions(
   user_session_id NUMBER
  ,user_id         NUMBER
  ,session_id      NUMBER
  ,CONSTRAINT user_sessions_pk PRIMARY KEY(user_session_id)
  ,CONSTRAINT user_session_user_id_fk FOREIGN KEY(user_id) REFERENCES users(user_id)
);

CREATE UNIQUE INDEX user_sessions_user_session_uk ON user_sessions(user_id, session_id);


CREATE TABLE toolbox_dirs_registered(
   dirname         VARCHAR2(100) NOT NULL
  ,user_session_id NUMBER(32)
  ,auto_register   CHAR(1)
  ,CONSTRAINT toolbox_dirs_registered_pk PRIMARY KEY(dirname)
  ,CONSTRAINT tdr_user_session_id_fk FOREIGN KEY(user_session_id) REFERENCES user_sessions(user_session_id)
);