RE: RE: serial type; race conditions

OK:
*    Transaction 1 commences, sets seed to seed + 1
*    Transaction 2 commences, sets seed to seed + 1
*    Transaction 1 inserts into some_table, selects seed (sequence now
has a hole)
*    Transaction 2 inserts into some_table, selects seed (same value as
just used by transaction 1)
*    The second transaction to commit will either create a duplicate
"seed" value, or roll back because of a unique constraint, still leaving a
hole in the sequence.

> -----Original Message-----
> From:    Gerald Gutierrez [SMTP:gutz@kalador.com]
> Sent:    Wednesday, April 04, 2001 7:24 PM
> To:    Jeff Eckermann; 'Andrew Perrin'
> Cc:    PgSQL-SQL
> Subject:    RE: [SQL] RE: serial type; race conditions
> 
> 
> It seems to just feel like conflicting requirements, so it's a tug-of-war.
> 
> I've always done it by doing all the processing I can and then, from
> inside
> a transaction, do
> 
> update seed from seed_table set seed=seed+1 where id='abc';
> insert into some_table values ((select seed from seed_table where
> id='abc'),
> other_stuff);
> 
> The processing would be concurrent and only the update & insert would be
> "serialized". It would be portable and shouldn't contain holes, but is
> slower than sequences.
> 
> 
> Gerald.
> 
> 
> -----Original Message-----
> From: pgsql-sql-owner@postgresql.org
> [mailto:pgsql-sql-owner@postgresql.org]On Behalf Of Jeff Eckermann
> Sent: Thursday, March 29, 2001 10:48 AM
> To: 'Andrew Perrin'
> Cc: PgSQL-SQL
> Subject: [SQL] 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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