Subject: Re: fetching unique pins in a high-transaction environment...
From: Scott Marlowe <smarlowe@g2switchworks.com>
To: Bobus <roblocke@gmail.com>
Cc: pgsql-sql@postgresql.org
In-Reply-To: <1162139808.031657.23340@f16g2000cwb.googlegroups.com>
References: <1162139808.031657.23340@f16g2000cwb.googlegroups.com>
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Date: Mon, 06 Nov 2006 13:21:43 -0600

On Sun, 2006-10-29 at 10:36, Bobus wrote:
> Hi,
> 
> I posted this question to the "general" forum, but then discovered this
> one which I think is more appropriate.  Apologies for the cross-post.
> 
> We are in the process of porting an application from SQL Server to
> PostgresQL.
> 
> We have a table which contains a bunch of prepaid PINs.  What is the
> best way to fetch the next available unique pin from the table in a
> high-traffic environment with lots of concurrent requests?
> 
> For example, our PINs table might look like this and contain thousands
> of records.  (FYI, the PIN numbers are generated by a third party and
> loaded into the table):
> 
> ID        PIN     USED_BY    DATE_USED
> ....
> 100     1864678198
> 101     7862517189
> 102     6356178381
> ....
> 
> 10 users request a pin at the same time.  What is the easiest/best way
> to ensure that the 10 users will get 10 unique pins, while eliminating
> any waiting?

Best practice, to me, is to do a couple things.  One, create a sequence
and set it to the first available pin number.  Let's say you have pins
available from the number 1 to 9999.  Create a default sequence, it'll
start on 1.  Then, select nextval('yourseqhere') and use that to fetch
the pin like so:

begin;
select nextval('yourseqhere'); --  store in a var
update pin set date_used=now() where id=$var and date_used IS NULL

If date_used is not null, then someone grabbed it from you.  Given that
we're grabbing them using a sequence, this is unlikely, but you never
know when things might go south.  

Otherwise you just reserved it.  Then grab it:

select pin from table where id=$var;
commit;

if a transaction fails, you might not use a pin, no big loss.  Better
than accidentally giving it out twice.

I'd wrap what I just wrote in a simple pl/pgsql script using security
definer and set the perms so ONLY the user defined function can get you
a new pin.