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From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Date: Mon, 23 Mar 2026 09:00:00 -0700
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Subject: Re: Introduce XID age based replication slot invalidation
To: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Cc: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>,
 John H <johnhyvr@gmail.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
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Hi,

On Fri, Mar 20, 2026 at 11:29=E2=80=AFPM SATYANARAYANA NARLAPURAM
<satyanarlapuram@gmail.com> wrote:
>
> Do you think we need different GUCs for catalog_xmin and xmin? If table b=
loat is a concern (not catalog bloat), then logical slots are not required =
to invalidate unless the cluster is close to wraparound.

IMO the main purpose of max_slot_xid_age is to prevent XID wraparound.
For bloat, I still think max_slot_wal_keep_size is the better choice.

Where max_slot_xid_age is really useful is when the vacuum can't
freeze because a replication slot (physical or logical) is holding
back the XID horizon and the system is getting close to wraparound.
Invalidating such a slot clears the way for vacuum. Setting
max_slot_xid_age above vacuum_failsafe_age allows vacuum to waste
cycles scanning tables it cannot freeze. Keeping max_slot_xid_age <=3D
vacuum_failsafe_age (default 1.6B) prevents this by invalidating the
slot before vacuum effort is wasted.

As far as XID wraparound is concerned, both xmin and catalog_xmin need
to be treated similarly. Either one can hold back freezing and push
the system toward wraparound. So I don't think we need separate GUCs
for xmin and catalog_xmin unless I'm missing something. One GUC
covering both keeps things simple.

>> I made the following design choice: try invalidating only once per
>> vacuum cycle, not per table. While this keeps the cost of checking
>> (incl. the XidGenLock contention) for invalidation to a minimum when
>> there are a large number of tables and replication slots, it can be
>> less effective when individual tables/indexes are large. Invalidating
>> during checkpoints can help to some extent with the large table/index
>> cases. But I'm open to thoughts on this.
>
> It may not solve the intent when the vacuum cycle is longer, which one ca=
n expect on a large database particularly when there is heavy bloat.

This design choice boils down to the following: a database instance
having either 1/ a large number of small tables or 2/ large tables.