Consistency Read user-defined function

1) Consistency Read user-defined function
1@@@@ Read consistency and user-defined functions
Note: two advises
      1.. SELECT ... FOR UPDATE [NOWAIT];
      2.. SET TRANSACTION READ ONLY; (need undo tbs)

  The read consistency model of the Oracle database is simple and clear: once I start a
  query, that query will only see data as it existed (was committed in the database) at the
  time the query was started. So if my query starts at 9:00 a.m. and runs for an hour, then
  even if another user comes along and changes data, my query will not see those changes.
 
  Yet unless you take special precautions with user-defined functions in your queries, it
  is quite possible that your query will violate (or, at least, appear to violate) the read
  consistency model of the Oracle database. To understand this issue, consider the following
  function and the query that calls it:
        FUNCTION total_sales (id_in IN account.account_id%TYPE)
          RETURN NUMBER
        IS
                CURSOR tot_cur
                IS
                SELECT SUM (sales) total
                  FROM orders
                 WHERE account_id = id_in
                   AND TO_CHAR (ordered_on, 'YYYY') = TO_CHAR (SYSDATE, 'YYYY');
                 --for update nowait
           tot_rec tot_cur%ROWTYPE;
        BEGIN
           OPEN tot_cur;
           FETCH tot_cur INTO tot_rec;
           RETURN tot_rec.total;
        END;
 
        SELECT name, total_sales (account_id)
        FROM account
        WHERE status = 'ACTIVE';
 
  The account table has 5 million active rows in it (a very successful enterprise!). The
  orders table has 20 million rows. I start the query at 10:00 a.m.; it takes about an hour
  to complete. At 10:45 a.m., somebody with the proper authority comes along, deletes
  all rows from the orders table, and performs a commit. According to the read consistency
  model of Oracle, the session running the query should not see all those deleted
  rows until the query completes. But the next time the total_sales function executes
  from within the query, it finds no order rows and returns NULL????nd will do so until
  the query completes.
 
  So if you are executing queries inside functions that are called inside SQL, you need to
  be acutely aware of read-consistency issues. If these functions are called in long-running
  queries or transactions, you will probably need to issue the following command to
  enforce read-consistency between SQL statements in the current transaction:
 
        SET TRANSACTION READ ONLY
 
  In this case, for read consistency to be possible, you need to ensure that you have
  sufficient undo tablespace.
 
referencing "Oracle PL/SQL Program Design 5th Edition"