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

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