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

On 11/2/18, Robert Haas <robertmhaas@gmail.com> wrote:
> On Fri, Nov 2, 2018 at 10:07 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Robert Haas <robertmhaas@gmail.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.

Correct.

> 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.

To be clear, no one suggested that. The patch has always had 8 or 10
as a starting point, and I've mentioned 4 and 8 as good possibilities
based on the COPY tests upthread. It was apparent I didn't need to
recompile a bunch of binaries with different thresholds. All I had to
do was compile with a threshold much larger than required, and then
test inserting into X number of pages, to simulate a threshold of X. I
increased X until I saw a regression. That's where the 32 came from,
sorry if that was misleading, in my head it was obvious. I'd be happy
test other scenarios. I'm not sure how to test redo -- seems more
difficult to get meaningful results than the normal case.

-John Naylor