Re: WIP: Avoid creation of the free space map for small tables

On Fri, Nov 2, 2018 at 10:07 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > That's not what I'm saying. If we don't have the FSM, we have to
> > check every page of the table. If there's a workload where that
> > happens a lot on a table that is just under the size threshold for
> > creating the FSM, then it's likely to be a worst case for this patch.
>
> Hmm, you're assuming something not in evidence: why would that be the
> algorithm?

I think it's in evidence, in the form of several messages mentioning a
flag called try_every_block.

Just checking the last page of the table doesn't sound like a good
idea to me. I think that will just lead to a lot of stupid bloat. It
seems likely that checking every page of the table is fine for npages
<= 3, and that would still be win in a very significant number of
cases, since lots of instances have many empty or tiny tables. I was
merely reacting to the suggestion that the approach should be used for
npages <= 32; that threshold sounds way too high.

--
Robert Haas
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