Thread: Doc limitation update proposal: include out-of-line OID usage per TOAST-ed columns

Hi -hackers,

I would like to ask if it wouldn't be good idea to copy the
https://wiki.postgresql.org/wiki/TOAST#Total_table_size_limit
discussion (out-of-line OID usage per TOAST-ed columns / potential
limitation) to the official "Appendix K. PostgreSQL Limits" with also
little bonus mentioning the "still searching for an unused OID in
relation" notice. Although it is pretty obvious information for some
and from commit 7fbcee1b2d5f1012c67942126881bd492e95077e and the
discussion in [1], I wonder if the information shouldn't be a little
more well known via the limitation (especially to steer people away
from designing very wide non-partitioned tables).

Regards,
-J.

[1] - https://www.postgresql.org/message-id/flat/16722-93043fb459a41073%40postgresql.org

On Fri, Apr 21, 2023 at 12:14 AM Nikita Malakhov <hukutoc@gmail.com> wrote:
> This limitation applies not only to wide tables - it also applies to tables where TOASTed values
> are updated very often. You would soon be out of available TOAST value ID because in case of
> high frequency updates autovacuum cleanup rate won't keep up with the updates. It is discussed
> in [1].
>
> On Fri, Apr 21, 2023 at 9:39 AM Jakub Wartak <jakub.wartak@enterprisedb.com> wrote:
>> I would like to ask if it wouldn't be good idea to copy the
>> https://wiki.postgresql.org/wiki/TOAST#Total_table_size_limit
>> discussion (out-of-line OID usage per TOAST-ed columns / potential
>> limitation) to the official "Appendix K. PostgreSQL Limits" with also
>> little bonus mentioning the "still searching for an unused OID in
>> relation" notice. Although it is pretty obvious information for some
>> and from commit 7fbcee1b2d5f1012c67942126881bd492e95077e and the
>> discussion in [1], I wonder if the information shouldn't be a little
>> more well known via the limitation (especially to steer people away
>> from designing very wide non-partitioned tables).
>>
>> [1] - https://www.postgresql.org/message-id/flat/16722-93043fb459a41073%40postgresql.org
>
> [1] https://www.postgresql.org/message-id/CAN-LCVPRvRzxeUdYdDCZ7UwZQs1NmZpqBUCd%3D%2BRdMPFTyt-bRQ%40mail.gmail.com

These 2 discussions show that it's a painful experience to run into
this problem, and that the hackers have ideas on how to fix it, but
those fixes haven't materialized for years. So I would say that, yes,
this info belongs in the hard-limits section, because who knows how
long it'll take this to be fixed.

Please submit a patch.

I anticipate that edits to Appendix K Postgres Limits will prompt
improving the note in there about the maximum column limit, That note
is too wordy, and sometimes confusing, especially for the audience
that it's written for: newcomers to Postgres ecosystem.

Best regards,
Gurjeet https://Gurje.et
http://aws.amazon.com

Hi,

>> These 2 discussions show that it's a painful experience to run into
>> this problem, and that the hackers have ideas on how to fix it, but
>> those fixes haven't materialized for years. So I would say that, yes,
>> this info belongs in the hard-limits section, because who knows how
>> long it'll take this to be fixed.
>>
>> Please submit a patch.
>>
> This is a production case for large databases with high update rates, but is mistaken
> with reaching table size limit, although size limit is processed correctly.
>
> The note on TOAST limitation does not mention that TOAST values are not actually
> updated on UPDATE operation - old value is marked as dead and new one is inserted,
> and dead values should be vacuumed before value OID could be reused. The worst
> is that the INSERT/UPDATE clause does not fail if there is no OID available - it is
> looped in an infinite loop of sorting out OIDs.

OK, so here is the documentation patch proposal. I've also added two
rows touching the subject of pg_largeobjects, as it is also related to
the OIDs topic. Please feel free to send adjusted patches.

Regards,
-J.

Hi John,

Thanks for your review. Here's v2 attached.

> -    <entry>partition keys</entry>
> -    <entry>32</entry>
> -    <entry>can be increased by recompiling <productname>PostgreSQL</productname></entry>
> +     <entry>partition keys</entry>
> +     <entry>32</entry>
> +     <entry>can be increased by recompiling <productname>PostgreSQL</productname></entry>
>
> Spurious whitespace.

Hopefully fixed, I've tried to align with the other entries tags.

> -     <entry>limited by the number of tuples that can fit onto 4,294,967,295 pages</entry>
> -     <entry></entry>
> +     <entry>limited by the number of tuples that can fit onto 4,294,967,295 pages or using up to 2^32 OIDs for
TOASTedvalues</entry>
 
> +     <entry>please see discussion below about OIDs</entry>
>
> I would keep the first as is, and change the second for consistency to "see note below on TOAST".

Fixed.

> Also, now that we have more than one note, we should make them more separate. That's something to discuss, no need to
doanything just yet.
 

OK.

> The new note needs a lot of editing to fit its new home. For starters:
>
> + <para>
> +  For every TOAST-ed columns
>
> column

Fixed.

> + (that is for field values wider than TOAST_TUPLE_TARGET
> +  [2040 bytes by default]), due to internal PostgreSQL implementation of using one
> +  shared global OID counter - today you cannot have more than
>
> + 2^32
>
> Perhaps it should match full numbers elsewhere in the page.

Fixed.

>
> +(unsigned integer;
>
> True but irrelevant.
>
> +  4 billion)
>
> Imprecise and redundant.

Removed both.

> + out-of-line values in a single table, because there would have to be
> +  duplicated OIDs in its TOAST table.
>
> The part after "because" should be left off.

Removed.

> +  Please note that that the limit of 2^32
> +  out-of-line TOAST values applies to the sum of both visible and invisible tuples.
>
> We didn't feel the need to mention this for normal tuples...

Right, but this somewhat points reader to the queue-like scenario
mentioned by Nikita.

> +  It is therefore crucial that the autovacuum manages to keep up with cleaning the
> +  bloat and free the unused OIDs.
> + </para>
>
> Out of place.

I have somewhat reworded it, again just to reference to the above.

> + <para>
> +  In practice, you want to have considerably less than that many TOASTed values
> +  per table, because as the OID space fills up the system might spend large
> +  amounts of time searching for the next free OID when it needs to generate a new
> +  out-of-line value.
>
> s/might spend large/will spend larger/ ?

Fixed.

> + After 1000000 failed attempts to get a free OID, a first log
> +  message is emitted "still searching for an unused OID in relation", but operation
> +  won't stop and will try to continue until it finds the free OID.
>
> Too much detail?

OK - partially removed.

> + Therefore,
> +  the OID shortages may (in very extreme cases) cause slowdowns to the
> +  INSERTs/UPDATE/COPY statements.
>
> s/may (in very extreme cases)/will eventually/

Fixed.

> + It's also worth emphasizing that
>
> Unnecessary.

Removed.

> + only field
> +  values wider than 2KB
>
> TOAST_TUPLE_TARGET

Good catch, fixed.

> + will consume TOAST OIDs in this way. So, in practice,
> +  reaching this limit would require many terabytes of data in a single table,
>
> It may be worth mentioning what Nikita said above about updates.

I've tried (with the above statement with visible and invisible tuples).

> +  especially if you have a wide range of value widths.
>
> I never understood this part.

I've changed it, but I wonder if the new "large number of wide
columns" isn't too ambiguous due to "large" (?)

> +   <row>
> +     <entry>large objects size</entry>
> +     <entry>subject to the same limitations as single <symbol>relation size</symbol></entry>
> +     <entry>LOs are stored in single pg_largeobjects relation</entry>
> +   </row>
>
> Are you under the impression that we can store a single large object up to table size? The limit is 4TB, as
documentedelsewhere.
 

I've wrongly put it, I've meant that pg_largeobject also consume OID
and as such are subject to 32TB limit.

>
> +   <row>
> +     <entry>large objects number</entry>
>
> "large objects per database"

Fixed.

> +     <entry>subject to the same limitations as <symbol>rows per table</symbol></entry>
>
> That implies table size is the only factor. Max OID is also a factor, which was your stated reason to include LOs
herein the first place.
 

Exactly..

Regards,
-Jakub Wartak.

On Wed, Apr 26, 2023 at 4:48 AM David Rowley <dgrowleyml@gmail.com> wrote:
>
> On Sun, 23 Apr 2023, 3:42 am Gurjeet Singh, <gurjeet@singh.im> wrote:
>>
>> I anticipate that edits to Appendix K Postgres Limits will prompt
>> improving the note in there about the maximum column limit, That note
>> is too wordy, and sometimes confusing, especially for the audience
>> that it's written for: newcomers to Postgres ecosystem.
>
>
> I doubt it, but feel free to submit a patch yourself which improves it without losing the information which the
paragraphis trying to convey. 

I could not think of a way to reduce the wordiness without losing
information. But since this page is usually consulted by those who are
new to Postgres, usually sent here by a search engine, I believe the
page can be improved for that audience, without losing much in terms
of accuracy.

I agree the information provided in the paragraph about max-columns is
pertinent. But since the limits section is most often consulted by
people migrating from other database systems (hence the claim that
they're new to the Postgres ecosystem), I imagine the terminology used
there may cause confusion for the reader. So my suggestion is to make
that paragraph, and perhaps even that page, use fewer hacker/internals
terms.

Technically, there may be a difference between table vs. relation, row
vs. tuple, and column vs. field. But using those terms, seemingly
interchangeably on that page does not help the reader. The page
neither describes the terms, nor links to their definitions, so a
reader is left with more questions than before. For example,

> rows per table:: limited by the number of tuples that can fit onto 4,294,967,295 pages

A newcomer> what's a tuple in this context, and how is it similar
to/different from a row?

Please see attached the proposed patch, which attempts to make that
language newcomer-friendly. The patch adds one link for TOAST, and
replaces Postgres-specific terms with generic ones.

PS: I've retained line boundaries, so that `git diff --color-words
doc/src/sgml/limits.sgml` would make it easy to see the changes.

Best regards,
Gurjeet
http://Gurje.et

> On 8 Aug 2023, at 12:31, John Naylor <john.naylor@enterprisedb.com> wrote:
>
> > > Also the shared counter is the cause of the slowdown, but not the reason for the numeric limit.
> >
> > Isn't it both? typedef Oid is unsigned int = 2^32, and according to GetNewOidWithIndex() logic if we exhaust the
wholeOID space it will hang indefinitely which has the same semantics as "being impossible"/permanent hang (?) 
>
> Looking again, I'm thinking the OID type size is more relevant for the first paragraph, and the shared/global aspect
ismore relevant for the second. 
>
> The last issue is how to separate the notes at the bottom, since there are now two topics.

Jakub, do you have plans to address this feedback? Is the CF entry still relevant?

Thanks!


Best regards, Andrey Borodin.

Hi Andrey,

On Thu, Mar 28, 2024 at 1:09 PM Andrey M. Borodin <x4mmm@yandex-team.ru> wrote:
>
>
>
> > On 8 Aug 2023, at 12:31, John Naylor <john.naylor@enterprisedb.com> wrote:
> >
> > > > Also the shared counter is the cause of the slowdown, but not the reason for the numeric limit.
> > >
> > > Isn't it both? typedef Oid is unsigned int = 2^32, and according to GetNewOidWithIndex() logic if we exhaust the
wholeOID space it will hang indefinitely which has the same semantics as "being impossible"/permanent hang (?) 
> >
> > Looking again, I'm thinking the OID type size is more relevant for the first paragraph, and the shared/global
aspectis more relevant for the second. 
> >
> > The last issue is how to separate the notes at the bottom, since there are now two topics.
>
> Jakub, do you have plans to address this feedback? Is the CF entry still relevant?

Yes; I've forgotten about this one and clearly I had problems
formulating it in proper shape to be accepted. I've moved it to the
next CF now as this is not critical and I would prefer to help current
timesenistive CF. Anyone is welcome to help amend the patch...

-J.

On Wed, Apr 3, 2024 at 4:59 AM Jakub Wartak
<jakub.wartak@enterprisedb.com> wrote:
> Yes; I've forgotten about this one and clearly I had problems
> formulating it in proper shape to be accepted. I've moved it to the
> next CF now as this is not critical and I would prefer to help current
> timesenistive CF. Anyone is welcome to help amend the patch...

I looked at your version and wrote something that is shorter and
doesn't touch any existing text. Here it is.

--
Robert Haas
EDB: http://www.enterprisedb.com

On Tue, May 14, 2024 at 8:19 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> I looked at your version and wrote something that is shorter and
> doesn't touch any existing text. Here it is.

Hi Robert, you are a real tactician here - thanks for whatever
references the original problem! :) Maybe just slight hint nearby
expensive (to me indicating a just a CPU problem?):

finding an OID that is still free can become expensive ->
finding an OID that is still free can become expensive, thus
significantly increasing INSERT/UPDATE response time.

? (this potentially makes it easier in future to pinpoint the user's
problem to the this exact limitation; but feel free to ignore that too
as I'm not attached to any of those versions)

-J.

On Mon, May 20, 2024 at 5:43 PM Jakub Wartak
<jakub.wartak@enterprisedb.com> wrote:
>
> On Tue, May 14, 2024 at 8:19 PM Robert Haas <robertmhaas@gmail.com> wrote:
> >
> > I looked at your version and wrote something that is shorter and
> > doesn't touch any existing text. Here it is.
>
> Hi Robert, you are a real tactician here - thanks for whatever
> references the original problem! :)

I like this text as well.

> Maybe just slight hint nearby
> expensive (to me indicating a just a CPU problem?):
>
> finding an OID that is still free can become expensive ->
> finding an OID that is still free can become expensive, thus
> significantly increasing INSERT/UPDATE response time.
>
> ? (this potentially makes it easier in future to pinpoint the user's
> problem to the this exact limitation; but feel free to ignore that too
> as I'm not attached to any of those versions)

Extra explicitness might be good. "Response time" seems like a
networking concept, so possibly ", in turn slowing down INSERT/UPDATE
statements." I'm inclined to commit that way in a couple days, barring
further comments.

PS: Sorry for the delay in looking at the latest messages

On Mon, Aug 12, 2024 at 11:01 AM John Naylor <johncnaylorls@gmail.com> wrote:
>
> Extra explicitness might be good. "Response time" seems like a
> networking concept, so possibly ", in turn slowing down INSERT/UPDATE
> statements." I'm inclined to commit that way in a couple days, barring
> further comments.

This is done.

On Tue, Aug 20, 2024 at 9:03 AM John Naylor <johncnaylorls@gmail.com> wrote:

> This is done.

Cool! Thanks John and Robert!  :)

-J.