Re: serial type; race conditions

I ditto what Bruce said - trying to get a true sequence without gaps is a
losing battle. Why don't you, instead, use a serial column as the real
sequence, and then a trigger that simply inserts max(foo) + 1 in a
different column? Then when you need to know the column, do something
like:

SELECT number_i_care_about FROM table WHERE serial_number =
currval('serial_number_seq');

ap

----------------------------------------------------------------------
Andrew J Perrin - Ph.D. Candidate, UC Berkeley, Dept. of Sociology
(Soon: Asst Professor of Sociology, U of North Carolina, Chapel Hill)
andrew_perrin(at)unc(dot)edu - http://www.unc.edu/~aperrin

On Thu, 29 Mar 2001, Bruce Momjian wrote:

> > How does currval work if you are not inside a transaction. I have
> > been experimenting with inserting into a table that has a sequence.
> > If the insert fails (not using a transaction) because of bad client input
> > then the next insert gets the proper next number in the sequence.
>
> If you are in a transaction, and the INSERT succeeds but the transaction
> rolls back, the sequence does not get reused. Each backend has a local
> variable that holds the most recent sequence assigned. That is how
> currval works.
>
> >
> > given sequence 1,2,3,4,5 exists
> > insert into table date 1/111/01 (obviously wrong) insert fails...
> > try again with good data, insert succeeds and gets number 6 in the
> > sequence.
> >
> > i'm getting what I want. A sequence number that does not increment
> > on a failed insert. However, how do I get the assigned sequence
> > number with currval when I am not using a transaction? What
> > happens when multiple users are inserting at the same time?
> >
> > I am trying to create a sequence with out any "missing" numbers. If
> > there is a failure to insert, and a sequence number is "taken". I want
> > the empty row.
> >
> > Thanks, .... it is getting clearer....
>
> You really can't use sequences with no gaps. Sequence numbers are not
> _held_ until commit because it would block other backends trying to get
> sequence numbers.
>
> --
> Bruce Momjian | http://candle.pha.pa.us
> pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
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