Re: Queuing query

On Mon, Sep 21, 2015 at 3:51 PM, Steve Crawford <
scrawford(at)pinpointresearch(dot)com> wrote:

> While awaiting the awesomeness of the upcoming "skip locked" feature in
> 9.5 I need to handle a work queue.
>
> Does anyone see any glaring issues or subtle nuances with the basic method
> below which combines CTEs with queue-handling methods posted by depesz, on
> the PG wiki and elsewhere.
>
> Note that it appears that there is the slight potential for a
> race-condition which would cause one worker to occasionally fail to get a
> record but the application code handles that issue fine.
>
> The work is sent to an externally hosted API which will ultimately reply
> to a callback API at our end so obviously there's a lot of other stuff in
> the system to update final results, recover from lost work, add to the
> queue, etc. I'm just asking about the sanity of the queue processing query
> itself:
>
> with next_up as (
> select
> the_id
> from
> queuetest
> where
> not sent_for_processing
> and pg_try_advisory_xact_lock(12345, the_id)
> order by
> the_priority
> limit 1 for update)
> update
> queuetest
> set
> sent_for_processing = true
> where
> the_id = (select the_id from next_up)
> returning
> the_work_to_do;
>

This will only be sane if the inner query can use an index to do the "order
by". Otherwise it is going to read every row in order to sort them, and
get the advisory lock on every row, and you will run out of shared memory.
Of course, if it were sorting each time it would probably be too slow
anyway.

And it has to be a partial index:

(the_priority) where not sent_for_processing

Because if you just have an index on the_priority, the sub select will
start getting inefficient once all the lowest numbered priority items are
marked as sent.

Also, you probably want to make sent_for_processing be some kind of token
or time stamp, to make it easier to detect lost work. In which case NULL
would mean not yet sent, so the partial index would be "where
sent_for_processing is null".

Cheers,

Jeff