Обсуждение: Re: unique sequences

> Rather, each time you wish to do an insert,
> query the existing data to see what the
> maximum value is existing in the database.
> Add 1 to that and use this as your new value.
> This is guaranteed to not have any holes in
> the sequence.

True, but alas it _doesn't_ guarantee uniqueness, since there's nothing to prohibit 2 or more back ends getting the
samemax() at the
 
same time...

Thinking off the top of my head, I'm not sure you can apply FOR UPDATE to an aggregate function, so there's probably no
solutionthere, either.
 



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I imagine it should if you create a unique index on the field.

Cheers,
Christopher Currie


----- Original Message -----
From: K Parker <kparker@eudoramail.com>
To: <pgsql-sql@hub.org>
Sent: Tuesday, June 27, 2000 1:52 AM
Subject: [SQL] Re: unique sequences


> > Rather, each time you wish to do an insert,
> > query the existing data to see what the
> > maximum value is existing in the database.
> > Add 1 to that and use this as your new value.
> > This is guaranteed to not have any holes in
> > the sequence.
>
> True, but alas it _doesn't_ guarantee uniqueness, since there's nothing to
prohibit 2 or more back ends getting the same max() at the
> same time...
>
> Thinking off the top of my head, I'm not sure you can apply FOR UPDATE to
an aggregate function, so there's probably no solution there, either.
>
>
>
> Join 18 million Eudora users by signing up for a free Eudora Web-Mail
account at http://www.eudoramail.com