Re: serial type; race conditions - Mailing list pgsql-sql

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@candle.pha.pa.us>
> To: jkakar@expressus.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@candle.pha.pa.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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