RE: serial type; race conditions

Probably just me: but I don't see the point.  Consider:
*    User 1 commences insert transaction: grabs nextval(sequence),
max(foo)
*    User 2 commences insert transaction: grabs nextval(sequence),
max(foo)
*    User 1 commits
*    User 2 commits (insert has sequence value one higher than for User
1, but same value for max(foo) + 1), or
*    If foo has a unique constraint, transaction 2 will roll back.

Either way, I don't see what has been gained.  All of the messages I have
read on this subject conclude with the same point: choice is to:
*    accept unique sequence with holes
*    accept loss of concurrency (as in the example above).

Or am I just missing the point?

> -----Original Message-----
> From:    Andrew Perrin [SMTP:aperrin@socrates.berkeley.edu]
> Sent:    Thursday, March 29, 2001 8:46 AM
> To:    pgsql@symcom.com
> Cc:    PgSQL-SQL
> Subject:    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@unc.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@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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