On Tue, Aug 21, 2012 at 10:32 AM, Craig James <cjames@xxxxxxxxxxxxxx> wrote:
> On Mon, Aug 20, 2012 at 5:10 PM, Tom Lane <tgl@xxxxxxxxxxxxx> wrote:
>> Craig James <cjames@xxxxxxxxxxxxxx> 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.
>>
>> regards, tom lane
>
> So here's what I came up with. I'm no PLPGSQL guru, but it seemed
> pretty straightforward.
>
> create or replace function nextval_block(bsize integer default 1)
> returns bigint as $nextval_block$
> declare
> bstart bigint;
> begin
> perform pg_advisory_lock(1);
> select into bstart nextval('my_seq');
> perform setval('my_seq', bstart + bsize, false);
> perform pg_advisory_unlock(1);
> return bstart;
> end;
> $nextval_block$ language plpgsql;
>
> As long as I ensure that every application uses nextval_block()
> instead of nextval() to access this sequence, I think this will do
> what I want.
>
> testdb=> select nextval_block();
> nextval_block
> ---------------
> 1
> (1 row)
>
> testdb=> select nextval_block();
> nextval_block
> ---------------
> 2
> (1 row)
>
>
> testdb=> select nextval_block(1000);
> nextval_block
> ---------------
> 3
> (1 row)
>
> testdb=> select nextval_block(1000);
> nextval_block
> ---------------
> 1003
> (1 row)
>
> testdb=> select nextval_block(1000);
> nextval_block
> ---------------
> 2003
> (1 row)
>
> Using pgsql's \timing directive, it says it's roughly 0.45 msec per
> request with the client and server are on the same machines, and 0.55
> msec per request when the client and server are different machines.
> Not bad.
If you also need to get only 1 id, in those cases you can sharelock
instead of full lock -- you can treat the case of blocksize=1
specially.
merlin
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