Re: Disabling Heap-Only Tuples
От | Thom Brown |
---|---|
Тема | Re: Disabling Heap-Only Tuples |
Дата | |
Msg-id | CAA-aLv4omW4L+R6C7m+93b8JHakSu5Hgs6w4-DfrLT4Xot15fg@mail.gmail.com обсуждение исходный текст |
Ответ на | Re: Disabling Heap-Only Tuples (Matthias van de Meent <boekewurm+postgres@gmail.com>) |
Ответы |
Re: Disabling Heap-Only Tuples
(Matthias van de Meent <boekewurm+postgres@gmail.com>)
|
Список | pgsql-hackers |
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); > 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. Thom
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