Re: Does setval(nextval()+N) generate unique blocks of IDs?

On Mon, Aug 20, 2012 at 6:59 PM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com> wrote:
> On Mon, Aug 20, 2012 at 6:10 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Craig James <cjames(at)emolecules(dot)com> writes:
>>> I want to do this:
>>
>>> select setval('object_id_seq', nextval('object_id_seq') + 1000, false);
>>
>>> Now suppose two processes do this simultaneously. Maybe they're in
>>> transactions, maybe they're not. Are they guaranteed to get distinct
>>> blocks of IDs?
>>
>> No, because the setval and the nextval are not indivisible.
>>
>>> Or is it possible that each will execute nextval() and
>>> get N and N+1 respectively, and then do setval() to N+1000 and N+1001,
>>> resulting in two overlapping blocks.
>>
>> Exactly.
>>
>>> If the answer is, "This won't work," then what's a better way to do this?
>>
>> AFAIK the only way at the moment is
>>
>> * acquire some advisory lock that by convention you use for this sequence
>> * advance the sequence
>> * release advisory lock
>>
>> There have been previous discussions of this type of problem, eg
>> http://archives.postgresql.org/pgsql-hackers/2011-09/msg01031.php
>> but the topic doesn't seem to have come up quite often enough to
>> motivate anybody to do anything about it. Your particular case could be
>> handled by a variant of nextval() with a number-of-times-to-advance
>> argument, but I'm not sure if that's enough for other scenarios.
>
> If the OP could live with large gaps in his sequence, he could set it
> to advance by say 1000 at a time, and then use the numbers in that gap
> freely. Just a thought.

Better yet set cache = 1000; here's an example:

create sequence a cache 1000;
T1: select nextval('a');
1
T2: select nextval('a');
1001
T1: select nextval('a');
2
T2: select nextval('a');
1002

and so on.

Now can he just select nextval('a'); 1000 times in a loop? Or would
he prefer another method.

I guess I'm kind of wondering which problem he's trying to solve.