Select for update doesn't work for me

I have a table that stores a set of numbers. These numbers, when used, should not be used again within 365 days. In addition, different process requesting a number should be split different numbers. Basically, a number must be used only once.

Here's the code I used to allocate 100 numbers. It is supposed to select a line, try to lock it using 'UPDATES'. If he is able to select, I guess that it is available and update the LASTMODTM. It's impossible to choose, this means another process has locked, and I will try the next number.

I've run 2 process running the code below together, expects that each process will get 2 unique series of numbers, but I'm left with exactly the same list of numbers for the two methods. I believe that SELECT for UPDATE is not working, that's why the two processes are able to choose the same line. Any advice for me?

DECLARE

v_nbr NUMBER_POOL. NBR % TYPE: = null;

CNT INTEGER: = 0;

v_nbrlist VARCHAR2 (32676): = ";

BEGIN

FOR x IN)

SELECT THE ROWID RID

OF NUMBER_POOL

WHERE

AND SYSDATE - LASTMODTM > 365

ORDER OF LASTMODTM, NBR

)

LOOP

BEGIN

-To lock the line so that it be referred to the other application

SELECT MAWB

IN v_nbr

OF NUMBER_POOL

WHERE ROWID = x.RID

FOR UPDATE NOWAIT.

EXCEPTION

-Impossible to line lock, this means that this number is locked by another process at the same time. Try the next nbr

WHILE OTHERS THEN

CONTINUE;

END;

UPDATE NUMBER_POOL

SET LASTMODTM = SYSTIMESTAMP

WHERE ROWID = x.RID;

CNT: = cnt + 1;

v_nbrlist: = v_nbrlist | ',' || v_nbr;

IF cnt = 100 THEN

DBMS_OUTPUT. Put_line (SUBSTR (v_nbrlist, 2));

EXIT;

END IF;

END LOOP;

END;

TKS all for your advice. I solved my problem

Sorry - but that does NOT solve your problem. It may "seem" to simple tests you run, but he does not take into account how locking and consistency reading actually works in Oracle.

It turns out that the 2 process has no conflict each othe at all, because the select sql loop took 0.2 sec, while 100 loops completed almost instantly. So the 2 process select almost at the same time, when first completed the process select, he received 100 numbers in a very short period of time, until the 2nd process is to select always. When the arrival of select 2nd process, the first process had already finished and committed the change. That is why the 2nd process does not meet lock exception.

Yes--but it's NOT the caue in the root of your problem. The problem is you are using TWO separate queries. The first request is to determine which rows to select, but it IS NOT lockingk these lines. This means that any other user can also select or even lock the rows that were selected only by the first user and the first query.

To fix this, I simply added "AND SYSDATE - LASTMODTM > 365" for numbers used will not be selected. The select for update query will be raise exception and then continue the loop to try the next number.

No - that will NOT fix the problem if you always use a separate SELECT statement for the first query.

A query, even a SELECT query, establishes sthe YVERT point-in-time for the data. This query second "select for update" is still being used by lines that were selected only, but not locked, the first query. This means that an another session/user may have selected some of these same rows in their first query, and then run the second query before and even during your attempt to execute your second query.

Tom Kyte shows, in the link I gave

In addition, there are some "read consistency problems" here.

The min (job_id) select where status = 0, which runs... It returns the number 100 (for example).

Update us this line - but have not committed yet.

Someone calls it, gets them min (job_id) select... you guessed it, 100. But they block on the update.

Now commit you and you have 100. They have now released and - well - update the SAME row...

for example: your logic does not work. You EF the same record N process sometimes.

The solution is to LOCK the lines with the first query so that no other session can perform DML on these lines. SKIP LOCKED clause in 11g is what you use.

See the example of Tom Kytes in its first reply in this thread;

https://asktom.Oracle.com/pls/asktom/f?p=100:11:0:P11_QUESTION_ID:2060739900346201280

Can you show how I can avoid using the QA and use SKIP LOCKED to elements of process simultaneously unpretentious? ...

Suppose you want to get the first line of a table that is not currently locked and corresponds to a unique key - or otherwise (single would not make sense, he would have one - and this would be trivial to determine whether it is locked or not, without the 'skip locked', you would use just nowait).

so the case of unique key is not remotely interesting, should not jump - needed ever - locked, just a nowait.

You come to question and search, for example:

OPS$ % ORA11GR2 tkyte > select empno

scott.emp 2
3 where job = 'CLERK '.
4.

EMPNO
----------
7369
7876
7900
7934

OPS$ % ORA11GR2 tkyte >
OPS$ % ORA11GR2 tkyte > declare
l_rec 2 scott.emp%rowtype;
slider 3 c is select * from scott.emp job where = 'CLERK' for update skip locked;
4 start
5. open c;
6 retrieve c in l_rec;
7. close c;
8 dbms_output.put_line (' I got empno = ' | l_rec.empno);
9 end;
10.
I myself empno = 7369

See the 'for UPDATE SKIP LOCKED' in the definition of the cursor? Which LOCKS the lines that are needed AND causes code pass all lines which can already be locked instead of throwing an exception as NOWAIT would.

Allows you to get some lines unlocked are available that meet your query and prevents the contention with other users.

NOTE: there is only ONE query.

Summary - This is your SELECT FIRST query that does NOT lock the lines are selected. It IS THE CAUSE of your problem. Your 'two query' solution ' will NOT work. Use the solution presented by Tom.

Tags: Database

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