Re: BUG #16045: vacuum_db crash and illegal memory alloc afterpg_upgrade from PG11 to PG12 - Mailing list pgsql-bugs

On Thu, Oct 10, 2019 at 04:14:20PM -0400, Tom Lane wrote:
>Tomas Vondra <tomas.vondra@2ndquadrant.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. 

OK.

> I get a hash semijoin to
>the CTE output, in all branches back to 8.4, on this trial query:
>
>explain
>with recursive sqlidoids(toid) as (
>select 'information_schema.sql_identifier'::pg_catalog.regtype as toid
>union
>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');
>

I think that's not quite sufficient - the problem is that we can have
domains and composite types on sql_identifier, in some arbitrary order.
And the recursive CTE won't handle that the way it's written - it will
miss domains on composite types containing sql_identifier. And we have
quite a few of them in the information schema, so maybe someone created
a domain on one of those (however unlikely it may seem).

I think this recursive CTE does it correctly:

WITH RECURSIVE oids AS (
    -- type itself
    SELECT 'information_schema.sql_identifier'::regtype AS oid
    UNION ALL
    SELECT * FROM (
        -- domains on the type
        WITH x AS (SELECT oid FROM oids)
        SELECT t.oid FROM pg_catalog.pg_type t, x WHERE typbasetype = x.oid AND typtype = 'd'
        UNION
        -- composite types containing the type
        SELECT t.oid FROM pg_catalog.pg_type t, pg_catalog.pg_class c, pg_catalog.pg_attribute a, x
        WHERE t.typtype = 'c' AND
              t.oid = c.reltype AND
              c.oid = a.attrelid AND
              a.atttypid = x.oid
    ) foo
) 

I had to use CTE within CTE, because the 'oids' can be referenced only
once, but we have two subqueries there. Maybe there's a better solution.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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