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

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?
--
Michael