Force Oracle to return TOP N rows with SKIP LOCKED

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.ID
FROM QueueTest qt
WHERE Locked IS NULL
ORDER BY PRIORITY
FOR 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_test
where rownum <= 5
order by c1 asc;

Which gives the results:

C1
--
6
7
8
9 
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
--
1
2
3
4
5

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 <= 5
for 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 on

declare
  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.

"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;