Re: possible race condition in trigger functions on insert operations?

On Tue, Nov 22, 2011 at 11:25 AM, Kenneth Tilton <ktilton(at)mcna(dot)net> wrote:
> Bit of a trigger NOOB Q:
>
> I am trying to use a trigger function to automatically populate new
> rows in a table with  a public ID of the form YYYY-NNN such that the
> 42nd row created in 2011 would get the ID "2011-042". Each row is
> associated via an iasid column with a row in an audit table that has a
> timestamp column called created. This works OK, but I am worried about
> two rows getting the same case_no if they come in at the same time
> (whatever that means):
>
> declare
>  case_yr integer;
>  yr_case_count bigint;
> begin
>  select date_part('year', created) into case_yr
>         from audit
>         where audit.sid = NEW.iasid;
>
>  select count(*) into yr_case_count
>    from fwa_case, audit
>    where fwa_case.iasid=audit.sid
>      and date_part('year', created) = case_yr;
>
>  NEW.case_no = to_char( case_yr, '9999' ) || '-' ||
> to_char(1+yr_case_count, 'FM000');
>  return NEW;
> end;
>
> Do I have to worry about this, or does ACID bail me out? If the
> former, what do I do? I am thinking first put a uniqueness constraint
> on the column and then figure out how to do retries in a trigger
> function.

ACID does not bail you out -- you've put no synchonization in to
prevent to concurrent counts coming at roughly the same time and
getting the same answer. A uniqueness constraint is definitely a good
idea. In terms of doing a gapless sequence generally, see here:
http://www.varlena.com/GeneralBits/130.php. Basically the general
idea is to keep a counter field somewhere that you lock and update.

merlin