Re: [Proposal] Expose internal MultiXact member count function for efficient monitoring

On Wed, Dec 17, 2025 at 9:49 AM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>
> On Sat, Dec 13, 2025 at 01:34:47PM -0600, Naga Appani wrote:
> > Documentation changes:
> > - Removed the NULL-return discussion from func-info.sgml, as the
> > statistics are now always available.
> > - Updated maintenance.sgml to clarify that exceeding the historical
> > 2^32 member limit no longer causes wraparound, but instead triggers
> > more aggressive vacuum activity for disk space management.
> >
> > I validated the behavior before and after cleanup.
> > The function correctly reports current usage (beyond the old limits) and
> > resets once multixacts are removed:
>
> + /*
> + * Calculate storage space for members. Members are stored in groups,
> + * with each group containing MULTIXACT_MEMBERS_PER_MEMBERGROUP members
> + * and taking MULTIXACT_MEMBERGROUP_SIZE bytes.
> + */
> + membersBytes = (int64) (members / MULTIXACT_MEMBERS_PER_MEMBERGROUP) *
> + MULTIXACT_MEMBERGROUP_SIZE;
>
> This is the key point of the patch internal logic. And there is one
> thing that I am wondering here. The amount of space taken by a number
> of members depends on the other compiled constants from
> multixact_internal.h. Hence, rather than calculate the amount of
> space taken by a set of members in some code hidden in the SQL
> function, could it be better to put that directly as a macro or an
> inline function in multixact_internal.h?

+1.

--
Best Wishes,
Ashutosh Bapat