Re: Vacuum ERRORs out considering freezing dead tuples from before OldestXmin

On Fri, Jul 26, 2024 at 1:27 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> On Mon, Jul 22, 2024 at 9:26 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > +   CREATE TABLE ${table1}(col1 int)
> > +       WITH (autovacuum_enabled=false, fillfactor=10);
> > +   INSERT INTO $table1 VALUES(7);
> > +   INSERT INTO $table1 SELECT generate_series(1, $nrows) % 3;
> > +   CREATE INDEX on ${table1}(col1);
> > +   UPDATE $table1 SET col1 = 3 WHERE col1 = 0;
> > +   INSERT INTO $table1 VALUES(7);
> >
> > These queries make sense to me; these make the radix tree wide and use
> > more nodes, instead of fattening lead nodes (i.e. the offset bitmap).
> > The $table1 has 18182 blocks and the statistics of radix tree shows:
> >
> > max_val = 65535
> > num_keys = 18182
> > height = 1, n4 = 0, n16 = 1, n32 = 0, n64 = 0, n256 = 72, leaves = 18182
> >
> > Which means that the height of the tree is 2 and we use the maximum
> > size node for all nodes except for 1 node.
>
> Do you have some kind of tool that prints this out for you? That would
> be really handy.

You can add '#define RT_DEBUG' for radix tree used in TidStore and
then call RT_STATS (e.g., local_ts_stats()).

>
> > I don't have any great idea to substantially reduce the total number
> > of tuples in the $table1. Probably we can use DELETE instead of UPDATE
> > to make garbage tuples (although I'm not sure it's okay for this
> > test). Which reduces the amount of WAL records from 11MB to 4MB and
> > would reduce the time to catch up. But I'm not sure how much it would
> > help. There might be ideas to trigger a two-round index vacuum with
> > fewer tuples but if the tests are too optimized for the current
> > TidStore, we will have to re-adjust them if the TidStore changes in
> > the future. So I think it's better and reliable to allow
> > maintenance_work_mem to be a lower value or use injection points
> > somehow.
>
> I think we can make improvements in overall time on master and 17 with
> the examples John provided later in the thread. However, I realized
> you are right about using a DELETE instead of an UPDATE. At some point
> in my development, I needed the UPDATE to satisfy some other aspect of
> the test. But that is no longer true. A DELETE works just as well as
> an UPDATE WRT the dead items and, as you point out, much less WAL is
> created and replay is much faster.
>
> I also realized I forgot to add 043_vacuum_horizon_floor.pl to
> src/test/recovery/meson.build in 16. I will post a patch here this
> weekend which changes the UPDATE to a DELETE in 14-16 (sped up the
> test by about 20% for me locally) and adds 043_vacuum_horizon_floor.pl
> to src/test/recovery/meson.build in 16. I'll plan to push it on Monday
> to save myself any weekend buildfarm embarrassment.
>
> As for 17 and master, I'm going to try out John's examples and see if
> it seems like it will be fast enough to commit to 17/master without
> lowering the maintenance_work_mem lower bound.

+1. Thanks.

Regards,

-- 
Masahiko Sawada
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