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

On Sun, Dec 28, 2025 at 9:51 PM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> So, here is what I have in mind, split into independent pieces:
> - Remove the existing type confusion with GetMultiXactInfo(), due to
> how things have always been done in MultiXactMemberFreezeThreshold().
> - Add macro MultiXactOffsetStorageSize(), to calculate the amount of
> space used between two offsets.
> - The main patch, with adjustments in comments, the test (no
> non-ASCII characters in that, please). One thing that was really
> surprising is that you did not consider ROLE_PG_READ_ALL_STATS. We
> expect all the stats information to be hidden if a role is not granted
> access to them, and this function should be no exception especially as
> it relates to disk space usage like database or tablespace size
> functions.
>
> Anyway, attached are all these updated pieces. The doc edits are what
> I have mentioned upthread, close to what you have suggested to me
> offline.
>
> Comments?
> --
> Michael

Thank you for patches, Michael! I've tested and everything works well:
- All patches apply cleanly
- Isolation test (multixact-stats) passes
- Function correctly reports stats under heavy load

Tested with significant multixact activity:
++++++++++++++++++++++++++++++++++++++++++++++
postgres=# \x
Expanded display is on.
postgres=# SELECT
to_char(num_mxids::bigint, 'FM999,999,999,999') AS num_mxids,
to_char(num_members::bigint, 'FM999,999,999,999') AS num_members,
to_char(members_size::bigint, 'FM999,999,999,999') AS members_size_bytes,
pg_size_pretty(members_size) AS members_size_pretty,
to_char(oldest_multixact::text::bigint, 'FM999,999,999,999') AS
oldest_multixact
FROM pg_get_multixact_stats();
-[ RECORD 1 ]-------+------------------
num_mxids | 235,095,556
num_members | 14,435,701,862
members_size_bytes | 72,178,509,300
members_size_pretty | 67 GB
oldest_multixact | 2
++++++++++++++++++++++++++++++++++++++++++++++

After cleanup, the function properly resets:
++++++++++++++++++++++++++++++++++++++++++++++
-[ RECORD 1 ]-------+-------------
num_mxids | 0
num_members | 0
members_size_bytes | 0
members_size_pretty | 0 bytes
oldest_multixact | 235,095,558
++++++++++++++++++++++++++++++++++++++++++++++

The oldest_multixact correctly advances to reflect the cleanup.

Thanks for adding the pg_read_all_stats privilege check!

I think this is ready for RFC.

Best regards,
Naga