Force Oracle to return TOP N rows with SKIP LOCKED

"From what I have seen, Oracle applies the WHERE predicate before determining what rows to skip."

Yup. It is the only possible way. You can't skip a row from a resultset until you have determined the resultset.

The answer is simply not to limit the number of rows returned by the SELECT statement. You can still use the FIRST_ROWS_n hints to direct the optimizer that you won't be grabbing the full data set.

The software calling the SELECT should only select the first n rows. In PL/SQL, it would be

DECLARE  CURSOR c_1 IS      SELECT /*+FIRST_ROWS_1*/ qt.ID    FROM QueueTest qt    WHERE Locked IS NULL    ORDER BY PRIORITY    FOR UPDATE SKIP LOCKED;BEGIN  OPEN c_1;  FETCH c_1 into ....  IF c_1%FOUND THEN     ...  END IF;  CLOSE c_1;END;

The solution Gary Meyers posted is about all I can think of, short of using AQ, which does all this for you and much more.

If you really want to avoid the PLSQL, you should be able to translate the PLSQL into Java JDBC calls. All you need to do is prepare the same SQL statement, execute it and then keep doing single row fetches on it (or N row fetches).

The Oracle documentation at http://download.oracle.com/docs/cd/B10501_01/java.920/a96654/resltset.htm#1023642 gives some clue how to do this at the statement level:

To set the fetch size for a query, call setFetchSize() on the statement object prior to executing the query. If you set the fetch size to N, then N rows are fetched with each trip to the database.

So you could code up something in Java that looks something like (in Pseudo code):

stmt = Prepare('SELECT /*+FIRST_ROWS_1*/ qt.IDFROM QueueTest qtWHERE Locked IS NULLORDER BY PRIORITYFOR UPDATE SKIP LOCKED');stmt.setFetchSize(10);stmt.execute();batch := stmt.fetch();foreach row in batch {  -- process row}commit (to free the locks from the update)stmt.close;

UPDATE

Based on the comments below, a suggestion was made to use ROWNUM to limit the results received, but that won't work in this case. Consider the example:

create table lock_test (c1 integer);begin  for i in 1..10 loop    insert into lock_test values (11 - i);  end loop;  commit;end;/

Now we have a table with 10 rows. Note that I have carefully inserted the rows in reverse order, the row containing 10 is first, then 9 etc.

Say you want the first 5 rows, ordered ascending - ie 1 to 5. Your first try is this:

select *from lock_testwhere rownum <= 5order by c1 asc;

Which gives the results:

C1--6789 10

That is clearly wrong, and is a mistake almost everyone makes! Look at the explain plan for the query:

| Id  | Operation           | Name      | Rows  | Bytes | Cost (%CPU)| Time     |---------------------------------------------------------------------------------|   0 | SELECT STATEMENT    |           |     5 |    65 |     4  (25)| 00:00:01 ||   1 |  SORT ORDER BY      |           |     5 |    65 |     4  (25)| 00:00:01 ||*  2 |   COUNT STOPKEY     |           |       |       |            |          ||   3 |    TABLE ACCESS FULL| LOCK_TEST |    10 |   130 |     3   (0)| 00:00:01 |---------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   2 - filter(ROWNUM<=5)

Oracle executes the plan from the bottom up - notice that the filter on rownum is carried out before the sort, Oracle takes the rows in the order it finds them (order they were inserted here { 10, 9, 8, 7, 6}), stops after it gets 5 rows, and then sorts that set.

So, to get the correct first 5 you need to do the sort first and then the order by using an inline view:

select * from(  select *  from lock_test  order by c1 asc)where rownum <= 5;C1--12345

Now, to finally get to the point - can you put a for update skip locked in the correct place?

select * from(  select *  from lock_test  order by c1 asc)where rownum <= 5for update skip locked;

This gives an error:

ORA-02014: cannot select FOR UPDATE from view with DISTINCT, GROUP BY, etc

Trying to move the for update into the view gives a syntax error:

select * from(  select *  from lock_test  order by c1 asc  for update skip locked)where rownum <= 5;

The only thing that will work is the following, which GIVES THE WRONG RESULT:

  select *  from lock_test  where rownum <= 5  order by c1 asc  for update skip locked;

Infact, if you run this query in session 1, and then run it again in session two, session two will give zero rows, which is really really wrong!

So what can you do? Open the cursor and fetch how many rows you want from it:

set serveroutput ondeclare  v_row lock_test%rowtype;  cursor c_lock_test  is  select c1  from lock_test  order by c1  for update skip locked;begin  open c_lock_test;  fetch c_lock_test into v_row;  dbms_output.put_line(v_row.c1);  close c_lock_test;end;/    

If you run that block in session 1, it will print out '1' as it got a locked the first row. Then run it again in session 2, and it will print '2' as it skipped row 1 and got the next free one.

This example is in PLSQL, but using the setFetchSize in Java you should be able get the exact same behaviour.

In your first session, when you execute:

SELECT qt.IDFROM QueueTest qtWHERE qt.ID IN (    SELECT ID    FROM        (SELECT ID FROM QueueTest WHERE Locked IS NULL ORDER BY Priority)    WHERE ROWNUM = 1)FOR UPDATE SKIP LOCKED

Your inner select attempt to grab only id=4 and lock it. This is successful because this single row is not yet locked.

In second session, your inner select STILL tries to grab ONLY id=4 and lock it. This is not successful because that single row is still locked by first session.

Now, if you updated the "locked" field in first session, the next session to run that select will grab id=3.

Basically, in your example you are depending on a flag that isn't being set. To use your locked flag, you probably mean to do something like:

1. select IDs you want based on some criteria.
2. Immediate update the locked flag = 1 for these IDs (if resource busy, another session beat you to this step for 1 or more IDs, goto 1 again)
3. Do whatever on these IDs
4. update the locked flag back to null

You can then use your select for update skip locked statement since your locked flag is being maintained.

Personally, I don't like all the updates to flags (your solution may require them for whatever reason), so I'd probably just try to select the IDs I want to update (by whatever criteria) in each session:

select * from queuetest where ... for update skip locked;

For example (in reality, my criteria wouldn't be based on a list of ids, but queuetest table is overly simplistic):

• sess 1: select * from queuetest whereid in (4,3) for update skip locked;

• sess 2: select * from queuetest whereid in (4,3,2) for update skip locked;

Here sess1 would lock 4,3 and sess2 would lock only 2.

You cannot to my knowledge do a top-n or use group_by/order_by etc in the select for update statement, you'll get a ORA-02014.