Re: Fix pg_upgrade to detect invalid logical replication slots on PG19

> On Apr 20, 2026, at 16:57, Lakshmi N <lakshmin(dot)jhs(at)gmail(dot)com> wrote:
>
> Hi Hackers,
>
> The PG19-optimized slot catchup query uses a CTE that filters on
> invalidation_reason IS NULL, then cross-joins it with the main slot
> query. When ALL logical slots in a database are invalid, the CTE
> returns zero rows, and the cross join produces an empty result set.
> This causes pg_upgrade to silently skip those slots entirely --
> neither detecting them as invalid (which should block the upgrade)
> nor attempting to migrate them.
>
> The pre-PG19 query path does not have this problem because it queries
> pg_replication_slots directly without a cross join. This may not impact
> upgrade to PG19 but will change the behavior for PG20 upgrade.
>
> Fix by changing the cross join to a LEFT JOIN,
> so that invalid slots still appear in the result set with NULL
> caught_up values.
>
> Regards,
> Lakshmi
> <0001-Fix-pg_upgrade-to-detect-invalid-logical-replication.patch>

This changes the inner join to a left join, which preserves the rows from pg_replication_slots. As check_caught_up is intentionally a singleton CTE because ORDER BY … LIMIT 1, so there is no row multiplication risk.

So, the patch looks good to me.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/