Re: BUG #16045: vacuum_db crash and illegal memory alloc after pg_upgrade from PG11 to PG12

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, buschmann(at)nidsa(dot)net, pgsql-bugs(at)lists(dot)postgresql(dot)org
Subject: Re: BUG #16045: vacuum_db crash and illegal memory alloc after pg_upgrade from PG11 to PG12
Date: 2019-10-10 20:14:20
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Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
> On Thu, Oct 10, 2019 at 10:19:12AM -0400, Tom Lane wrote:
>> To identify such domains, I think we'd need something like
>> WHERE attypid IN (recursive-WITH-query), which makes me nervous.
>> We did support those starting with 8.4, which is as far back as
>> pg_upgrade will go, so in theory it should work. But I think we
>> had bugs with such cases in old releases. Do we want to assume
>> that the source server has been updated enough to avoid any such
>> bugs? The expense of such a query might be daunting, too.

> For the query cost, I think we can assume the domain hierarchies are not
> particularly deep (in practice I'd expect just domains directly on the
> sql_identifier type). And I doubt people are using that very widely,
> it's probably more like this report - ad-hoc CTAS, so just a couple of
> items. So I wouldn't expect it to be a huge deal in most cases. But even
> if it takes a second or two, it's a one-time cost.

What I was worried about was the planner possibly trying to apply the
atttypid restriction as a scan qual using a subplan, which might be rather
awful. But it doesn't look like that happens. I get a hash semijoin to
the CTE output, in all branches back to 8.4, on this trial query:

with recursive sqlidoids(toid) as (
select 'information_schema.sql_identifier'::pg_catalog.regtype as toid
select oid from pg_catalog.pg_type, sqlidoids
where typtype = 'd' and typbasetype = sqlidoids.toid
SELECT n.nspname, c.relname, a.attname
FROM pg_catalog.pg_class c,
pg_catalog.pg_namespace n,
pg_catalog.pg_attribute a
WHERE c.oid = a.attrelid AND
NOT a.attisdropped AND
a.atttypid in (select toid from sqlidoids) AND
c.relkind IN ('r','v','i') and
c.relnamespace = n.oid AND
n.nspname !~ '^pg_temp_' AND
n.nspname !~ '^pg_toast_temp_' AND
n.nspname NOT IN ('pg_catalog', 'information_schema');

regards, tom lane

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