Re: easy task: concurrent select-updates

Kevin McConnell wrote:
>> CREATE OR REPLACE FUNCTION public.getmsg() RETURNS integer LANGUAGE plpgsql
>> AS $function$
>> declare
>> rec record;
>> begin
>> for rec in select id from msg where busy = false order by id loop
>> update msg set busy = true where id = rec.id and busy = false;
>> if found then
>> return rec.id;
>> end if;
>> end loop;
>> return -1;
>> end;
>> $function$
>
> I think you could also do something roughly similar in a statement by
> using a RETURNING clause on the update, such as:
>
> update msg set busy = true where id = (select min(id) from msg where
> busy = false) returning *;
>
> Cheers,
> Kevin
>

I had thought of that, but you'd need to add one thing, in the update ' and busy = false ', cuz two people may get the same id from the select min(id).

update msg set busy = true where busy = false and id = (select min(id) from msg where busy = false) returning *;

but then you'd have to fire it over-and-over until you actually got a row updated.

Seemed easer to put the loop in function, then you can:

select id from getmsg();

-Andy