Re: pgsql: Increase upper limit for vacuum_cleanup_index_scale_factor

On Mon, Apr 15, 2019 at 11:57 AM Alexander Korotkov
<a.korotkov@postgrespro.ru> wrote:
>
> Hi!
>
> On Sun, Apr 14, 2019 at 11:00 PM Piotr Stefaniak
> <postgres@piotr-stefaniak.me> wrote:
> > On 26/06/2018 14.35, Alexander Korotkov wrote:
> > > Increase upper limit for vacuum_cleanup_index_scale_factor
> > >
> > > Upper limits for vacuum_cleanup_index_scale_factor GUC and reloption
> > > were initially set to 100.0 in 857f9c36.  However, after further
> > > discussion, it appears that some users like to disable B-tree cleanup
> > > index scan completely (assuming there are no deleted pages).
> > >
> > > vacuum_cleanup_index_scale_factor is used barely to protect against
> > > stalled index statistics.  And after detailed consideration it appears
> > > that risk of stalled index statistics is low.  And it would be nice to
> > > allow advanced users setting higher values of
> > > vacuum_cleanup_index_scale_factor.  So, set upper limit for these
> > > GUC and reloption to DBL_MAX.
> >
> > UB Sanitizer points out that prev_num_heap_tuples is sometimes 0,
> > leading to division by 0 in
> >                         (info->num_heap_tuples - prev_num_heap_tuples) /
> >                         prev_num_heap_tuples >= cleanup_scale_factor)
> > which are currently lines 839-840 in nbtree.c.

Thank you pointing out it.

> >
> > Attaching my idea of a fix.
>
> Thank you for noticing.  BTW, I've more trivial idea for fixing this: replace
> prev_num_heap_tuples < 0
> with
> prev_num_heap_tuples <= 0
>
> If prev_num_heap_tuples == 0, subsequent part of expression isn't
> evaluated because result is known to be true.  And I think it's right
> to don't skip cleanup when prev_num_heap_tuples == 0.

+1. It looks good to me.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center