Re: serial type; race conditions

If you are looking to have every number accounted for, something like this
will work:

INSERT INTO table (serial_col) SELECT nextval('seq_serial_col');

UPDATE table SET foo = 'bar' , ... WHERE serial_col = (SELECT
currval('seq_serial_col'));

then, if the update fails, the number will be accounted for in the
table (Note that you could not use not null on any of the columns).

Michael Fork - CCNA - MCP - A+
Network Support - Toledo Internet Access - Toledo Ohio

On Thu, 29 Mar 2001, postgresql 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.
>
> 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....
>
> Ted
>
>
> -----Original Message-----
> From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
> To: jkakar(at)expressus(dot)com
> Date: Mon, 26 Mar 2001 16:47:37 -0500 (EST)
> Subject: Re: [SQL] serial type; race conditions
>
> > > Hi,
> > >
> > > I'm using serial fields to generate IDs for almost all object in my
> > > database. I insert an empty row, get the CURRVAL() of the
> sequence
> > > and then update to that value.
> > >
> > > I had understood (and now, I can't find the reference to back this
> > up)
> > > that serial is implemented in such a way that race conditions
> between
> > > DB connections can't happen.
> > >
> > > Is this true?
> >
> > Safe. See FAQ item. currval is for your backend only.
> >
> > --
> > Bruce Momjian | http://candle.pha.pa.us
> > pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
> > + If your life is a hard drive, | 830 Blythe Avenue
> > + Christ can be your backup. | Drexel Hill, Pennsylvania
> > 19026
> >
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