Re: serial type; race conditions

Micheal,

Thanks, I was thinking that something like this should work.
However, I am having a problem with it. here is what I am doing.

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

first I needed to add parens around the select statement. The
results are not what I expected. If I executed this a few times, when I
looked at the table what I saw was:

serial_col | seq_serial_col
1 | 2
3 | 4
5 | 6

etc.
I had thought I would do the insert, grab the currval of transaction
passing it back to my app. commit, then do an update. I can not
seem to get the seq to work.

Ted

-----Original Message-----
From: Michael Fork <mfork(at)toledolink(dot)com>
To: postgresql <pgsql(at)symcom(dot)com>
Date: Thu, 29 Mar 2001 10:04:46 -0500 (EST)
Subject: Re: [SQL] 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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