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

On Wed, Jul 24, 2024 at 2:42 PM John Naylor <johncnaylorls(at)gmail(dot)com> wrote:
> As for lowering the limit, we've experimented with 256kB here:
>
> https://www.postgresql.org/message-id/CANWCAZZUTvZ3LsYpauYQVzcEZXZ7Qe+9ntnHgYZDTWxPuL++zA@mail.gmail.com
>
> As I mention there, going lower than that would need a small amount of
> reorganization in the radix tree. Not difficult -- the thing I'm
> concerned about is that we'd likely need to document a separate
> minimum for DSA, since that behaves strangely with 256kB and might not
> work at all lower than that.

For experimentation, here's a rough patch (really two, squashed
together for now) that allows m_w_m to go down to 64kB.

drop table if exists test;
create table test (a int) with (autovacuum_enabled=false, fillfactor=10);
insert into test (a) select i from generate_series(1,2000) i;
create index on test (a);
update test set a = a + 1;

set maintenance_work_mem = '64kB';
vacuum (verbose) test;

INFO: vacuuming "john.public.test"
INFO: finished vacuuming "john.public.test": index scans: 3
pages: 0 removed, 91 remain, 91 scanned (100.00% of total)

The advantage with this is that we don't need to care about
MEMORY_CONTEXT_CHECKING or 32/64 bit-ness, since allocating a single
large node will immediately blow the limit, and that will happen
fairly quickly regardless. I suspect going this low will not work with
dynamic shared memory and if so would need a warning comment.