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  1. Remove replication slot advice from MultiXact wraparound hints

  1. BUG #18876: HINT messages for mxid wrap-around say "drop stale slots", but that may not be appropriate

    PG Bug reporting form <noreply@postgresql.org> — 2025-04-04T03:05:12Z

    The following bug has been logged on the website:

Bug reference:      18876
Logged by:          TAKATSUKA Haruka
Email address:      harukat@sraoss.co.jp
PostgreSQL version: 17.4
Operating system:   any
Description:        

In src/backend/access/transam/multixact.c, there are the following hint
messages:

  "Execute a database-wide VACUUM in that database.\n"
  "You might also need to commit or roll back old prepared transactions, or
drop stale replication slots."

  "To avoid MultiXactId assignment failures, execute a database-wide VACUUM
in that database.\n"
  "You might also need to commit or roll back old prepared transactions, or
drop stale replication slots."

I think that their "drop stale replication slots" is not appropriate. 

Because NewRelminMxid is determined by GetOldestMultiXactId() and its
comment says:
   * Return the oldest MultiXactId that's still possibly still seen as live
by  
   * any running transaction.  Older ones might still exist on disk, but
they no
   * longer have any running member transaction.

Thus, the presence of an old mxid in a tuple pending removal by a slot is
not considered to affect it.

In addition, as far as I have tested, leaving the old inactive replication
slot does not cause mxid_age(relminmxid) not to decrease after VACUUM.

Thanks,

  2. Re: BUG #18876: HINT messages for mxid wrap-around say "drop stale slots", but that may not be appropriate

    Fujii Masao <masao.fujii@gmail.com> — 2026-06-11T03:31:28Z

    On Fri, Apr 4, 2025 at 10:30 PM PG Bug reporting form
<noreply@postgresql.org> wrote:
>
> The following bug has been logged on the website:
>
> Bug reference:      18876
> Logged by:          TAKATSUKA Haruka
> Email address:      harukat@sraoss.co.jp
> PostgreSQL version: 17.4
> Operating system:   any
> Description:
>
> In src/backend/access/transam/multixact.c, there are the following hint
> messages:
>
>   "Execute a database-wide VACUUM in that database.\n"
>   "You might also need to commit or roll back old prepared transactions, or
> drop stale replication slots."
>
>   "To avoid MultiXactId assignment failures, execute a database-wide VACUUM
> in that database.\n"
>   "You might also need to commit or roll back old prepared transactions, or
> drop stale replication slots."
>
> I think that their "drop stale replication slots" is not appropriate.
>
> Because NewRelminMxid is determined by GetOldestMultiXactId() and its
> comment says:
>    * Return the oldest MultiXactId that's still possibly still seen as live
> by
>    * any running transaction.  Older ones might still exist on disk, but
> they no
>    * longer have any running member transaction.
>
> Thus, the presence of an old mxid in a tuple pending removal by a slot is
> not considered to affect it.
>
> In addition, as far as I have tested, leaving the old inactive replication
> slot does not cause mxid_age(relminmxid) not to decrease after VACUUM.

I think you're right. So I prepared and attached a patch that refines
the MXID wraparound hint messages. The patch also updates the
documentation to clarify the relationship between replication slots and
MXID wraparound.

Thought?

Regards,

-- 
Fujii Masao

  3. Re: BUG #18876: HINT messages for mxid wrap-around say "drop stale slots", but that may not be appropriate

    TAKATSUKA Haruka <harukat@sraoss.co.jp> — 2026-06-11T04:29:10Z

    Fujii-san

Thank you for incorporating my feedback into the code and documentation.

with best regards,
Haruka Takatsuka / SRA OSS K.K.


On Thu, 11 Jun 2026 12:31:28 +0900
Fujii Masao <masao.fujii@gmail.com> wrote:

> On Fri, Apr 4, 2025 at 10:30 PM PG Bug reporting form
> <noreply@postgresql.org> wrote:
> >
> > The following bug has been logged on the website:
> >
> > Bug reference:      18876
> > Logged by:          TAKATSUKA Haruka
> > Email address:      harukat@sraoss.co.jp
> > PostgreSQL version: 17.4
> > Operating system:   any
> > Description:
> >
> > In src/backend/access/transam/multixact.c, there are the following hint
> > messages:
> >
> >   "Execute a database-wide VACUUM in that database.\n"
> >   "You might also need to commit or roll back old prepared transactions, or
> > drop stale replication slots."
> >
> >   "To avoid MultiXactId assignment failures, execute a database-wide VACUUM
> > in that database.\n"
> >   "You might also need to commit or roll back old prepared transactions, or
> > drop stale replication slots."
> >
> > I think that their "drop stale replication slots" is not appropriate.
> >
> > Because NewRelminMxid is determined by GetOldestMultiXactId() and its
> > comment says:
> >    * Return the oldest MultiXactId that's still possibly still seen as live
> > by
> >    * any running transaction.  Older ones might still exist on disk, but
> > they no
> >    * longer have any running member transaction.
> >
> > Thus, the presence of an old mxid in a tuple pending removal by a slot is
> > not considered to affect it.
> >
> > In addition, as far as I have tested, leaving the old inactive replication
> > slot does not cause mxid_age(relminmxid) not to decrease after VACUUM.
> 
> I think you're right. So I prepared and attached a patch that refines
> the MXID wraparound hint messages. The patch also updates the
> documentation to clarify the relationship between replication slots and
> MXID wraparound.
> 
> Thought?
> 
> Regards,
> 
> -- 
> Fujii Masao