Re: [HACKERS] Concurrent ALTER SEQUENCE RESTART Regression

On Tue, May 23, 2017 at 11:25 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2017-05-23 22:47:07 -0400, Robert Haas wrote:
>> On Mon, May 22, 2017 at 11:42 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> > Ooops.
>> >
>> > Two issues: Firstly, we get a value smaller than seqmin, obviously
>> > that's not ok. But even if we'd error out, it'd imo still not be ok,
>> > because we have a command that behaves partially transactionally
>> > (keeping the seqmax/min transactionally), partially not (keeping the
>> > current sequence state at -9).
>>
>> I don't really agree that this is broken.
>
> Just a quick clarification question: You did notice that nextval() in S1
> after the rollback returned -9, despite seqmin being 0? I can see
> erroring out being acceptable, but returning flat out wrong values....?

I did see that. I'm unclear what you think it should do instead. I
mean, it could notice that cur_val < min_val and return min_val, but
that doesn't have a very good chance of being correct either. I
suppose the aborted transaction could try to restore the old cur_val
when it rolls back, but that's clearly the wrong thing when there's no
DDL involved (plus I'm not sure it would even be safe to try to do
that). Do you have an idea?

--
Robert Haas
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