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From: Chao Li <[email protected]>
To: Lakshmi N <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: Fix pg_upgrade to detect invalid logical replication slots on PG19
Date: Wed, 22 Apr 2026 17:13:01 +0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <CA+3i_M8eT6j8_cBHkYykV-SXCxbmAxpVSKptjDVq+MFtpT-Paw@mail.gmail.com>
References: <CA+3i_M8eT6j8_cBHkYykV-SXCxbmAxpVSKptjDVq+MFtpT-Paw@mail.gmail.com>



> On Apr 20, 2026, at 16:57, Lakshmi N <[email protected]> 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/









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