Re: Remaining case where reltuples can become distorted across multiple VACUUM operations

On Mon, 8 Aug 2022 at 17:26, Peter Geoghegan <pg@bowt.ie> wrote:
>
> On Mon, Aug 8, 2022 at 8:14 AM Matthias van de Meent
> <boekewurm+postgres@gmail.com> wrote:
> > I do not have intimate knowledge of this code, but shouldn't we also
> > add some sefety guarantees like the following in these blocks? Right
> > now, we'll keep underestimating the table size even when we know that
> > the count is incorrect.
> >
> > if (scanned_tuples > old_rel_tuples)
> >     return some_weighted_scanned_tuples;
>
> Not sure what you mean -- we do something very much like that already.
>
> We take the existing tuple density, and assume that that hasn't
> changed for any unscanned pages -- that is used to build a total
> number of tuples for the unscanned pages. Then we add the number of
> live tuples/scanned_tuples that the vacuumlazy.c caller just
> encountered on scanned_pages. That's often where the final reltuples
> value comes from.

Indeed we often apply this, but not always. It is the default case,
but never applied in the special cases.

For example, if currently the measured 2% of the pages contains more
than 100% of the previous count of tuples, or with your patch the last
page contains more than 100% of the previous count of the tuples, that
new count is ignored, which seems silly considering that the vacuum
count is supposed to be authorative.

Kind regards,

Matthias van de Meent