Re: Relation extension scalability

On 11/03/16 22:29, Robert Haas wrote:
> On Thu, Mar 10, 2016 at 8:54 PM, Petr Jelinek <petr@2ndquadrant.com> wrote:
>> I am not talking about extension locks, the lock queue can be long because
>> there is concurrent DDL for example and then once DDL finishes suddenly 100
>> connections that tried to insert into table will try to get extension lock
>> and this will add 2000 new pages when much fewer was actually needed. I
>> guess that's fine as it's corner case and it's only 16MB even in such
>> extreme case.
>
> I don't really understand this part about concurrent DDL.  If there
> were concurrent DDL going on, presumably other backends would be
> blocked on the relation lock, not the relation extension lock - and it
> doesn't seem likely that you'd often have a huge pile-up of inserters
> waiting on concurrent DDL.  But I guess it could happen.
>

Yeah I was thinking about the latter part and as I said it's very rare 
case, but I did see something similar couple of times in the wild. It's 
not objection against committing this patch though, in fact I think it 
can be committed as is.

--   Petr Jelinek                  http://www.2ndQuadrant.com/  PostgreSQL Development, 24x7 Support, Training &
Services