Re: display offset along with block number in vacuum errors

On Fri, Aug 7, 2020 at 8:10 AM Masahiko Sawada
<masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
>
> On Fri, 7 Aug 2020 at 10:49, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Thu, Aug 6, 2020 at 7:51 PM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:
> > >
> > > On Thu, Aug 06, 2020 at 07:39:21PM +0530, Amit Kapila wrote:
> > > > On Wed, Jul 29, 2020 at 1:09 AM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:
> > > > >
> > > > >
> > > > > lazy_check_needs_freeze iterates over blocks and this patch changes it to
> > > > > update vacrelstats. I think it should do what
> > > > > lazy_{vacuum/cleanup}_heap/page/index do and call update_vacuum_error_info() at
> > > > > its beginning (even though only the offset is changed), and then
> > > > > restore_vacuum_error_info() at its end (to "revert back" to the item number it
> > > > > started with).
> > > > >
> > > >
> > > > I see that Mahendra has changed patch as per this suggestion but I am
> > > > not convinced that it is a good idea to sprinkle
> > > > update_vacuum_error_info()/restore_vacuum_error_info() at places more
> > > > than required. I see that it might look a bit clean from the
> > > > perspective that if tomorrow we use the function
> > > > lazy_check_needs_freeze() for a different purpose then we don't need
> > > > to worry about the wrong phase information. If we are worried about
> > > > that then we should have an assert in that function to ensure that the
> > > > current phase is VACUUM_ERRCB_PHASE_SCAN_HEAP.
> > >
> > > The motivation was to restore the offnum, which is set to Invalid at the start
> > > of lazy_scan_heap(), and then set valid within lazy_check_needs_freeze, but
> > > should be restored or re-set to Invalid when returns to lazy_scan_heap(). If
> > > you think it's important, we could just set vacrelstats->offnum = Invalid
> > > before returning,
> > >
> >
> > Yeah, I would prefer that and probably a comment to indicate why we
> > are doing that.
>
> +1
>
> I'm concerned that we call the update and restore in
> heap_page_is_all_visible(). Unlike lazy_check_needs_freeze(), this
> function is called for every vacuumed page. I commented that if we
> want to update the offset number during iterating tuples in the
> function we should change the phase to SCAN_HEAP at the beginning of
> the function because it's actually not vacuuming.
>

AFAICS, heap_page_is_all_visible() is called from only one place and
that is lazy_vacuum_page(), so I think if there is any error in
heap_page_is_all_visible(), it should be considered as VACUUM_HEAP
phase error.

But if the error is
> unlikely to happen within the function I think we can avoid updating
> the offset number and phase to avoid performance overhead.
>

I am not sure we can guarantee that and even if it is true today one
can add an error in that path in future. But I feel we can keep the
phase as VACUUM_HEAP.

--
With Regards,
Amit Kapila.