Re: Correct usage of FOR UPDATE?

Hey Joe,

You can wrap the given query in the PL/pgSQL function,
catch the exception and return NULL if it occurs. Please see
http://www.postgresql.org/docs/9.0/static/plpgsql-control-structures.html#PLPGSQL-ERROR-TRAPPING

2010/12/9 Joe Carr <joe(dot)carr(at)gmail(dot)com>

> Hello,
> I have a simple table-based queue system, and I'd looking for some advice
> on improving my dequeue function. it boils down to:
>
> SELECT id
> FROM queue
> WHERE <whereclause>
> FOR UPDATE NOWAIT;
>
> which works well, in that no item gets dequeued more that once. The issue,
> however is that when a contention occurs, the error "could not obtain lock
> on row in relation" is raised.
>
> Is there a way that I can rewrite this so that when a contention occurs,
> there is no error? I would like the "winning" process to lock the row, and
> the "losing" process to select null rather than raise the rowlock error.
>
> thanks beforehand for any help!
>

--
// Dmitriy.