Problem in working on 2 sql session :: data become inconsistence

Question

I follow below steps in sequence and land-up into inconsistence data.
Suppose I have a table EMP (EMPNO , SAL ) with record (1, 100);
Session-1
I execute below command and deducted 70 from sal. This Deduction is valid
since sal > 70 . I didn’t COMMIT.
DECLARE
   n   NUMBER (4);
BEGIN
   SELECT sal
     INTO n
     FROM emp
    WHERE empno = 1;
   IF n - 70 > 0
   THEN
      UPDATE emp
         SET sal = sal - 70
       WHERE empno = 1;
   END IF;
END;
--Before committing data in session-1 , I executed below code in session-2 ::
--deducted 50 from sal.
--This Deduction is valid for second session since sal > 50
DECLARE
   n   NUMBER (4);
BEGIN
   SELECT sal
     INTO n
     FROM emp
    WHERE empno = 1;
   IF n - 50 > 0
   THEN
      UPDATE emp
         SET sal = sal - 50
       WHERE empno = 1 ;
   END IF;
END;
The session-2 hangs (in waiting state) since session-1 has locked the record.
As soon as session-1 releases the record (i.e. commit data in session-1) session-2 completes,
which actually give me WRONG value .Problem::
Despite EMP=1 has sal = 100 , 2 transaction occur first for 70 and other for 50.
When I use::
SELECT ….. for UPDATE nowait , I am able to get rid of this.
Can I use NOWAIT option directly in my UPDATE statement?
--I am working on oracle 9i                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       

Answer 1

J99 wrote:
I follow below steps in sequence and land-up into inconsistence data.
.. cut ..
As soon as session-1 releases the record (i.e. commit data in session-1) session-2 completes,
which actually give me WRONG value .
.. cut ..
SELECT ….. for UPDATE nowait , I am able to get rid of this. I agree with the others, it is giving the correct value for what the code is doing. The "wrong"ness comes from how you have written your code.
The point of using a "FOR UPDATE" on your select statement is to say "I'm selecting this data with the intention of possibly updating the same" which therefore tells Oracle to hold off actually selecting the data until the table(s) are in a state where you won't get an inconsistency. Currently, you are letting oracle select the data regardless (i.e. not showing any intention of updating it) and then you are updating the data based on a "dirty" read. It is this dirty reading technique that is at fault if you are really intending to update based on what you select.
The NOWAIT option shows your intention to update the data, but also says you don't want to wait for other transactions to complete. In this scenario Oracle will read any rows from the table that have not yet been locked by other transactions and allow you to continue your processing for those rows. This is fine if you are working specifically on data identified by primary keys, but could lead to indeterminate processing if it's a more global select/update scenario without specific keys; i.e. you couldn't be sure which of the transactions will process which rows of data as it will come down to a timing issue of which transaction gets any particular row first.