Re: The Free Space Map: Problems and Opportunities

On Wed, Aug 18, 2021 at 3:58 PM Peter Geoghegan <pg@bowt.ie> wrote:
> On Tue, Aug 17, 2021 at 5:31 PM Peter Geoghegan <pg@bowt.ie> wrote:
> > > Now what's the threshold? 20 out of 100? 50? 80?
> >
> > I'm not going to pretend to know the answer. But I will point out that
> > one DB system whose heap fill factor defaults to 90 seems to have a
> > symmetric setting for the "open up page again" point -- the default
> > for that is 40. Not sure that that really matters to us, but that does
> > seem pretty low to me. It's very sticky indeed.
>
> Correction: it's actually 60, not 40.
>
> It's true that the actual default is 40, but it works the other way
> around relative to Postgres (as does the related fill factor like
> setting, which defaults to 90 instead of 100). And so we would think
> of this other "open up closed page once again" setting as having a
> default of 60. (Or perhaps we'd think of it as having a default that
> is 2/3 of the closely related fill factor setting's default.)

I don't know whether 60 is optimal or not, but it doesn't seem crazy.

-- 
Robert Haas
EDB: http://www.enterprisedb.com