Re: Application locking - Mailing list pgsql-general

We want to make sure no two examiners are working on the same case at the same time, where the cases are found by searching on certain criteria with limit 1 to get the "next case".

A naive approach would be (in a stored procedure):

        next_case_id := null;

select id into next_case_id 

from cases c

where unfinished = true

and not exists (select 1 from table_lock

where table_name = 'case' and row_id = c.id)

limit 1;

if found then

insert into table_lock (table_name, row_id) values ('case', next_case_id);

end if;

return next_case_id;

I suspect it would be possible for two users to get the same case locked that way. Yes?

If so, would adding "for update" to the initial select prevent a second caller to block on their select until the first caller had written out the lock, effectively preventing two callers from locking the same case?

If not, can we do better by bundling it all into one statement?:

        with nc as (select c.id clm_id from cases c

where unfinished = true

and not exists (select 1 from table_lock

where table_name = 'case' and row_id = c.id)

limit 1) , ic as (insert into rdf (iasid, s,p,oint) select 42, user_id, 'started-editing', clm_id from nc returning oint locked) select locked from ic limit 1 into locked_id; return locked_id;

If I am all wet, is their a reliable way to achieve this?

Thx, kt

--
Kenneth Tilton

Director of Software Development

MCNA Dental Plans
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