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From: Masahiko Sawada <sawada.mshk@gmail.com>
Date: Tue, 18 May 2021 16:09:37 +0900
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Subject: Re: Testing autovacuum wraparound (including failsafe)
To: Peter Geoghegan <pg@bowt.ie>
Cc: Andres Freund <andres@anarazel.de>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>
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On Tue, May 18, 2021 at 2:46 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Tue, May 18, 2021 at 2:42 PM Peter Geoghegan <pg@bowt.ie> wrote:
> >
> > On Mon, May 17, 2021 at 10:29 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > > +1 to fix this. Are you already working on fixing this? If not, I'll
> > > post a patch.
> >
> > I posted a patch recently (last Thursday my time). Perhaps you can review it?
>
> Oh, I missed that the patch includes that fix. I'll review the patch.
>

I've reviewed the patch. Here is one comment:

    if (vacrel->num_index_scans == 0 &&
-       vacrel->rel_pages <= FAILSAFE_MIN_PAGES)
+       vacrel->rel_pages <= FAILSAFE_EVERY_PAGES)
        return false;

Since there is the condition "vacrel->num_index_scans == 0" we could
enter the failsafe mode even if the table is less than 4GB, if we
enter lazy_check_wraparound_failsafe() after executing more than one
index scan. Whereas a vacuum on the table that is less than 4GB and
has no index never enters the failsafe mode. I think we can remove
this condition since I don't see the reason why we don't allow to
enter the failsafe mode only when the first-time index scan in the
case of such tables. What do you think?

Regards,

-- 
Masahiko Sawada
EDB:  https://www.enterprisedb.com/