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

On Tue, Aug 21, 2012 at 1:41 AM, Scott Marlowe <scott.marlowe@gmail.com> wrote:
> On Mon, Aug 20, 2012 at 7:06 PM, Scott Marlowe <scott.marlowe@gmail.com> wrote:
>> On Mon, Aug 20, 2012 at 6:59 PM, Scott Marlowe <scott.marlowe@gmail.com> wrote:
>>> On Mon, Aug 20, 2012 at 6:10 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>>> Craig James <cjames@emolecules.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

sometimes I hate my laptops touchpad.  Ran something similar in php
got similar performance.  By comparison, running select 1 instead of
nextval() took ~0.160s to run.