Re: pg_upgrade failing for 200+ million Large Objects

On Sat, Jul 27, 2024 at 1:37 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Justin Pryzby <pryzby@telsasoft.com> writes:
> > On Fri, Jul 26, 2024 at 10:53:30PM +0300, Alexander Korotkov wrote:
> >> It would be nice to identify such cases and check which memory contexts are
> >> growing and why.
>
> > I reproduced the problem with this schema:
>
> > SELECT format('CREATE TABLE p(i int, %s) PARTITION BY RANGE(i)', array_to_string(a, ', ')) FROM (SELECT
array_agg(format('i%sint', i))a FROM generate_series(1,999)i); 
> > SELECT format('CREATE TABLE t%s PARTITION OF p FOR VALUES FROM (%s)TO(%s)', i,i,i+1) FROM generate_series(1,999)i;
>
> > This used over 4 GB of RAM.
>
> Interesting.  This doesn't bloat particularly much in a regular
> pg_restore, even with --transaction-size=1000; but it does in
> pg_upgrade, as you say.  I found that the bloat was occurring
> during these long sequences of UPDATE commands issued by pg_upgrade:
>
> -- For binary upgrade, recreate inherited column.
> UPDATE pg_catalog.pg_attribute
> SET attislocal = false
> WHERE attname = 'i'
>   AND attrelid = '\"public\".\"t139\"'::pg_catalog.regclass;
>
> -- For binary upgrade, recreate inherited column.
> UPDATE pg_catalog.pg_attribute
> SET attislocal = false
> WHERE attname = 'i1'
>   AND attrelid = '\"public\".\"t139\"'::pg_catalog.regclass;
>
> -- For binary upgrade, recreate inherited column.
> UPDATE pg_catalog.pg_attribute
> SET attislocal = false
> WHERE attname = 'i2'
>   AND attrelid = '\"public\".\"t139\"'::pg_catalog.regclass;
>
> I think the problem is basically that each one of these commands
> causes a relcache inval, for which we can't reclaim space right
> away, so that we end up consuming O(N^2) cache space for an
> N-column inherited table.

I was about to report the same.

> It's fairly easy to fix things so that this example doesn't cause
> that to happen: we just need to issue these updates as one command
> not N commands per table.  See attached.  However, I fear this should
> just be considered a draft, because the other code for binary upgrade
> in the immediate vicinity is just as aggressively stupid and
> unoptimized as this bit, and can probably also be driven to O(N^2)
> behavior with enough CHECK constraints etc.  We've gone out of our way
> to make ALTER TABLE capable of handling many updates to a table's DDL
> in one command, but whoever wrote this code appears not to have read
> that memo, or at least to have believed that performance of pg_upgrade
> isn't of concern.

I was thinking about counting actual number of queries, not TOC
entries for transaction number as a more universal solution.  But that
would require usage of psql_scan() or writing simpler alternative for
this particular purpose.  That looks quite annoying.  What do you
think?


------
Regards,
Alexander Korotkov
Supabase