Atomic UPDATE .. SELECT in Postgres

Question

I'm building a queuing mechanism of sorts. There are rows of data that need processing, and a status flag. I'm using an update .. returning clause to manage it:

UPDATE stuff
SET computed = 'working'
WHERE id = (SELECT id from STUFF WHERE computed IS NULL LIMIT 1)
RETURNING * 

Is the nested select part the same lock as the update, or do I have a race condition here? If so, does the inner select need to be a select for update?

Answer 1

While Erwin's suggestion is possibly the simplest way to get correct behavior (so long as you retry your transaction if you get an exception with SQLSTATE of 40001), queuing applications by their nature tend to work better with requests blocking for a chance to take their turn at the queue than with the PostgreSQL implementation of SERIALIZABLE transactions, which allows higher concurrency and is somewhat more "optimistic" about the chances of collision.

The example query in the question, as it stands, in the default READ COMMITTED transaction isolation level would allow two (or more) concurrent connections to both "claim" the same row from the queue. What will happen is this:

• T1 starts and gets as far as locking the row in the UPDATE phase.
• T2 overlaps T1 in execution time and attempts to update that row. It blocks pending the COMMIT or ROLLBACK of T1.
• T1 commits, having successfully "claimed" the row.
• T2 tries to update the row, finds that T1 already has, looks for the new version of the row, finds that it still satisfies the selection criteria (which is just that id matches), and also "claims" the row.

It can be modified to work correctly (if you are using a version of PostgreSQL which allows the FOR UPDATE clause in a subquery). Just add FOR UPDATE to the end of the subquery which selects the id, and this will happen:

• T1 starts and now locks the row before selecting the id.
• T2 overlaps T1 in execution time and blocks while trying to select an id, pending the COMMIT or ROLLBACK of T1.
• T1 commits, having successfully "claimed" the row.
• By the time T2 is able to read the row to see the id, it sees that it has been claimed, so it finds the next available id.

At the REPEATABLE READ or SERIALIZABLE transaction isolation level, the write conflict would throw an error, which you could catch and determine was a serialization failure based on the SQLSTATE, and retry.

If you generally want SERIALIZABLE transactions but you want to avoid retries in the queuing area, you might be able to accomplish that by using an advisory lock.

Answer 2

If you are the only user, the query should be fine. In particular, there is no race condition or deadlock within the query itself (between the outer query and the subquery). The manual:

However, a transaction never conflicts with itself.

For concurrent use, the matter may be more complicated. You would be on the safe side with SERIALIZABLE transaction mode:

BEGIN ISOLATION LEVEL SERIALIZABLE;
UPDATE stuff
SET    computed = 'working'
WHERE  id = (SELECT id FROM stuff WHERE computed IS NULL LIMIT 1)
RETURNING * 
COMMIT;

You need to prepare for serialization failures and retry your query in such a case.

But I am not entirely sure if this isn't overkill. I'll ask @kgrittn to stop by .. he is the expert with concurrency and serializable transactions ..

And he did. :)

Best of both worlds

Run the query in default transaction mode READ COMMITTED.

For Postgres 9.5 or later use FOR UPDATE SKIP LOCKED. See:

• Postgres UPDATE … LIMIT 1

For older versions recheck the condition computed IS NULL explicitly in the outer UPDATE:

UPDATE stuff
SET    computed = 'working'
WHERE  id = (SELECT id FROM stuff WHERE computed IS NULL LIMIT 1)
AND   computed IS NULL;

As @kgrittn's advised in the comment to his answer, this query could come up empty, without having done anything, in the (unlikely) case it got intertwined with a concurrent transaction.

Therefore, it would work much like the first variant in transaction mode SERIALIZABLE, you would have to retry - just without the performance penalty.

The only problem: While the conflict is very unlikely because the window of opportunity is just so tiny, it can happen under heavy load. You could not tell for sure whether there are finally no more rows left.

If that does not matter (like in your case), you are done here.
If it does, to be absolutely sure, start one more query with explicit locking after you get an empty result. If this comes up empty, you are done. If not, continue.
In plpgsql it could look like this:

LOOP
   UPDATE stuff
   SET    computed = 'working'
   WHERE  id = (SELECT id FROM stuff WHERE computed IS NULL
                LIMIT 1 FOR UPDATE SKIP LOCKED);  -- pg 9.5+
   -- WHERE  id = (SELECT id FROM stuff WHERE computed IS NULL LIMIT 1)
   -- AND    computed IS NULL; -- pg 9.4-

   CONTINUE WHEN FOUND;  -- continue outside loop, may be a nested loop

   UPDATE stuff
   SET    computed = 'working'
   WHERE  id = (SELECT id FROM stuff WHERE computed IS NULL
                LIMIT 1 FOR UPDATE);

   EXIT WHEN NOT FOUND;  -- exit function (end)
END LOOP;

That should give you the best of both worlds: performance and reliability.