Re: Concurrent ALTER SEQUENCE RESTART Regression

On Thu, Apr 27, 2017 at 4:10 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On April 27, 2017 12:06:55 AM PDT, Michael Paquier <michael(dot)paquier(at)gmail(dot)com> wrote:
>>On Thu, Apr 27, 2017 at 3:23 PM, Andres Freund <andres(at)anarazel(dot)de>
>>wrote:
>>> More fun:
>>>
>>> A: CREATE SEQUENCE someseq;
>>> A: BEGIN;
>>> A: ALTER SEQUENCE someseq MAXVALUE 10;
>>> B: SELECT nextval('someseq') FROM generate_series(1, 1000);
>>>
>>> => ignores maxvalue
>>
>>Well, for this one that's because the catalog change is
>>transactional...
>
> Or because the locking model is borked.

The operation actually relies heavily on the fact that the exclusive
lock on the buffer of pg_sequence is hold until the end of the catalog
update. And using heap_inplace_update() seems mandatory to me as the
metadata update should be non-transactional, giving the attached. I
have added some isolation tests. Thoughts? The attached makes HEAD map
with the pre-9.6 behavior.
--
Michael