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

On Mon, Aug 20, 2012 at 7:06 PM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com> wrote:
> 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.

Made a sequence:
create sequence a;

then ran a one line
select nextval('a');
against it 1000 times from bash, i.e. the worst vase performance scenario:

time for ((i=0;i<1000;i++));do psql -f t1 > /dev/null;done

real 1m1.978s
user 0m41.999s
sys 0m12.277s

then I ran it a singe time on a file with 1000 select nextvals:

time psql -f t1000 > /dev/null

real 0m0.486s
user 0m0.112s
sys 0m0.036s

Then I recreated sequence a:

create sequence a cache 1000;

and ran it again:

time psql -f t1000 > /dev/null

real 0m0.293s
user 0m0.120s
sys 0m0.024s

I'd imagine in a real programming oangua