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@toledolink.com>
To: postgresql <pgsql@symcom.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@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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