Re: pgsql: Avoid creation of the free space map for small heap relations, t

On Mon, Feb 25, 2019 at 11:14 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> I wrote:
> > Amit Kapila <akapila(at)postgresql(dot)org> writes:
> >> Avoid creation of the free space map for small heap relations, take 2.
>
> > I think this patch still has some issues.
>
> Just out of curiosity ... how can it possibly be even a little bit sane
> that fsm_local_map is a single static data structure, without even any
> indication of which table it is for?
>

It is just for finding a free block among a few blocks of relation.
We clear this when we have found a block with enough free space, when
we extend the relation, or on transaction abort. So, I think we don't
need any per table information.

> If, somehow, there's a rational argument for that design, why is it
> not explained in freespace.c? The most charitable interpretation of
> what I see there is that it's fatally undercommented.
>

There is some explanation in freespace.c and in README
(src/backend/storage/freespace/README). I think we should add some
more information where this data structure (FSMLocalMap) is declared.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com