Thread

Re: Disabling Heap-Only Tuples

Thom Brown <thom@linux.com> — 2023-07-05T17:54:53Z

On Wed, 5 Jul 2023 at 18:05, Matthias van de Meent
<boekewurm+postgres@gmail.com> wrote:
>
> On Wed, 5 Jul 2023 at 14:39, Thom Brown <thom@linux.com> wrote:
> >
> > On Wed, 5 Jul 2023 at 13:12, Matthias van de Meent
> > <boekewurm+postgres@gmail.com> wrote:
> > >
> > > On Wed, 5 Jul 2023 at 13:03, Thom Brown <thom@linux.com> wrote:
> > > >
> > > > On Wed, 5 Jul 2023 at 11:57, Matthias van de Meent
> > > > <boekewurm+postgres@gmail.com> wrote:
> > > > >
> > > > > On Wed, 5 Jul 2023 at 12:45, Thom Brown <thom@linux.com> wrote:
> > > > > > Heap-Only Tuple (HOT) updates are a significant performance
> > > > > > enhancement, as they prevent unnecessary page writes. However, HOT
> > > > > > comes with a caveat: it means that if we have lots of available space
> > > > > > earlier on in the relation, it can only be used for new tuples or in
> > > > > > cases where there's insufficient space on a page for an UPDATE to use
> > > > > > HOT.
> > > > > >
> > > > > > This mechanism limits our options for condensing tables, forcing us to
> > > > > > resort to methods like running VACUUM FULL/CLUSTER or using external
> > > > > > tools like pg_repack. These either require exclusive locks (which will
> > > > > > be a deal-breaker on large tables on a production system), or there's
> > > > > > risks involved. Of course we can always flood pages with new versions
> > > > > > of a row until it's forced onto an early page, but that shouldn't be
> > > > > > necessary.
> > > > > >
> > > > > > Considering these trade-offs, I'd like to propose an option to allow
> > > > > > superusers to disable HOT on tables. The intent is to trade some
> > > > > > performance benefits for the ability to reduce the size of a table
> > > > > > without the typical locking associated with it.
> > > > >
> > > > > Interesting use case, but I think that disabling HOT would be missing
> > > > > the forest for the trees. I think that a feature that disables
> > > > > block-local updates for pages > some offset would be a better solution
> > > > > to your issue: Normal updates also prefer the new tuple to be stored
> > > > > in the same pages as the old tuple if at all possible, so disabling
> > > > > HOT wouldn't solve the issue of tuples residing in the tail of your
> > > > > table - at least not while there is still empty space in those pages.
> > > >
> > > > Hmm... I see your point.  It's when an UPDATE isn't going to land on
> > > > the same page that it relocates to the earlier available page.  So I
> > > > guess I'm after whatever mechanism would allow that to happen reliably
> > > > and predictably.
> > > >
> > > > So $subject should really be "Allow forcing UPDATEs off the same page".
> > >
> > > You'd probably want to do that only for a certain range of the table -
> > > for a table with 1GB of data and 3GB of bloat there is no good reason
> > > to force page-crossing updates in the first 1GB of the table - all
> > > tuples of the table will eventually reside there, so why would you
> > > take a performance penalty and move the tuples from inside that range
> > > to inside that same range?
> >
> > I'm thinking more of a case of:
> >
> > <magic to stop UPDATES from landing on same page>
> >
> > UPDATE bigtable
> > SET primary key = primary key
> > WHERE ctid IN (
> >     SELECT ctid
> >     FROM bigtable
> >     ORDER BY ctid DESC
> >     LIMIT 100000);
>
> So what were you thinking of? A session GUC? A table option?

Both.

> The benefit of a table option is that it is retained across sessions
> and thus allows tables that get enough updates to eventually get to a
> cleaner state. The main downside of such a table option is that it
> requires a temporary table-level lock to update the parameter.

Yes, but the maintenance window to make such a change would be extremely brief.

> The benefit of a session GUC is that you can set it without impacting
> other sessions, but the downside is that you need to do the
> maintenance in that session, and risk that cascading updates to other
> tables (e.g. through AFTER UPDATE triggers) are also impacted by this
> non-local update GUC.
>
> > > Something else to note: Indexes would suffer some (large?) amount of
> > > bloat in this process, as you would be updating a lot of tuples
> > > without the HOT optimization, thus increasing the work to be done by
> > > VACUUM.
> > > This may result in more bloat in indexes than what you get back from
> > > shrinking the table.
> >
> > This could be the case, but I guess indexes are expendable to an
> > extent, unlike tables.
>
> I don't think that's accurate - index rebuilds are quite expensive.
> But, that's besides the point of this thread.
>
> Somewhat related: did you consider using pg_repack instead of this
> potential feature?

pg_repack isn't exactly innocuous, and can leave potentially the
database in an irrevocable state.  Plus, if disk space is an issue, it
doesn't help.

Thom