Re: [SQL] abusing an aggregate funct

On Sun, 15 Nov 1998, Marc Howard Zuckman wrote:

> On Sun, 15 Nov 1998, Postgres DBA wrote:
>
> >
> >
> > On Sun, 15 Nov 1998, M Simms wrote:
> >
> > > > Unfortunately, solution using sequences is not so good unless you don't
> > > > suppose to use access to that table from some simultaneously running
> > > > sessions. The problem is that every sessions accessing this additional
> > > > sequence  with nextval()  will get its own pool of values for the
> > > > sequence, so early or later you'll get some gaps in records numbering
> > > > because of at least on of two reasons:
> > >
> > > >From the manpage of create sequence
> > >
> > >
> > >        Low-level locking is used to enable multiple  simultaneous
> > >        calls to a generator.
> > >
> > > Doesnt this mean it will work for this task?
> > >
> > >
> >
> > Sure, you can access one sequence from a couple of sessions at once but
> > it the only thing is guaranteed is UNIQUENESS of values you get from
> > nestval() across one sequence. Although nobody can promise you that there
> > will no gaps... There are lots of reasons, I mentioned just some of them
> > (see my previous posting in this thread)
> >
> > Aleksey
> >
> >
> >
> >
>
> My understanding is that if you do not enable the cache feature
> of the sequence, there will be no gaps.  Of course, disabling the
> cache slows the transaction.

You are extremly right:-) Nevertheless, don't forget of another problem:
you CAN'T call curval() before nextval(). So, every time you want to check
the maximum row number in the table you should increase sequence counter
using nextval() -- here is real source of gaps, and I don't know if it is
possible to overcome it :_( The only idea I have for now is to check
last_value field of the sequence instead of calling curval()...

But again, what to do with nextval() calls during rollbacked transaction??

Aleksey.