Re: BUG #5027: SQL query error?

On Tue, Sep 1, 2009 at 9:51 AM, Dmitry<master(at)hsdesign(dot)ru> wrote:
>
> The following bug has been logged online:
>
> Bug reference:      5027
> Logged by:          Dmitry
> Email address:      master(at)hsdesign(dot)ru
> PostgreSQL version: 8.3.5
> Operating system:   ALT Linux
> Description:        SQL query error?
> Details:
>
> Hello,
>
> I try to make user rating by this query:
>
> "BEGIN;
> ALTER SEQUENCE service.rate_seq RESTART 1;
> UPDATE service.user u
>   SET rate = sr.rate
> FROM (
>   SELECT user_id, nextval('service.rate_seq') as rate
>   FROM
>     (
>      SELECT user_id, score FROM service.user
>      ORDER BY score DESC
>     ) a
>   ) sr
> WHERE u.user_id = sr.user_id;
> END;"
>
> all work fine, but if while this query is executing another process do
> "UPDATE service.user SET score=score+1" i get wrong sequence value; for
> example
> if user count = 55000 i can get 512321 in sequence ;(
> why? query error?

Because sequences are non-transactional. See the description of
nextval(), here:

http://www.postgresql.org/docs/current/interactive/functions-sequence.html

...Robert