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

On 2025-08-22 09:28, torikoshia wrote:
> On 2025-08-20 13:27, Naga Appani wrote:
>
> Thanks for working on this!
>
>> On Tue, Aug 19, 2025 at 1:32 AM Michael Paquier <michael(at)paquier(dot)xyz>
>> wrote:
>>> FWIW, I think that you should be a bit more careful before sending
>>> updated patch sets. You have missed an extra point I have raised
>>> upthread about the refactoring pieces: the switch from
>>> ReadMultiXactCounts() to GetMultiXactInfo() can be done in a patch of
>>> its own.
>>>
>>> So I have extracted this part from your latest patch, and applied it
>>> independently of the SQL function business. Now we are in an
>>> advantageous position on HEAD: even if we do not conclude about the
>>> SQL function to show the mxact numbers and offsets, we have the
>>> function that gives an access to the data you are looking for. In
>>> short, it is now possible to provide an equivalent of the feature you
>>> want outside of core. Not saying that the patch cannot be useful,
>>> but
>>> such refactoring pieces open more possibilities, and offer a cleaner
>>> commit history with less churn in the main patches.
>>> --
>>
>> Thanks for the review and separating the refactoring into its own
>> commit.
>> Point taken on being more careful when sending updated patch sets.
>> I’ll make sure to keep
>> refactoring and SQL layer changes clearly separated going forward.
>>
>> Attached is v6, rebased on top of HEAD. This version is limited to the
>> SQL function only.
>>
>
> diff --git a/doc/src/sgml/maintenance.sgml
> b/doc/src/sgml/maintenance.sgml
> index e7a9f58c015..6f0e8d7c10a 100644
> --- a/doc/src/sgml/maintenance.sgml
> +++ b/doc/src/sgml/maintenance.sgml
> @@ -813,12 +813,56 @@ HINT: Execute a database-wide VACUUM in that
> database.
> <para>
> As a safety device, an aggressive vacuum scan will
> occur for any table whose multixact-age is greater than <xref
> - linkend="guc-autovacuum-multixact-freeze-max-age"/>. Also, if
> the
> - storage occupied by multixacts members exceeds about 10GB,
> aggressive vacuum
> + linkend="guc-autovacuum-multixact-freeze-max-age"/>. Also, if the
> number
> + of members created exceeds approximately 2^31 entries, aggressive
> vacuum
> scans will occur more often for all tables, starting with those
> that
>
> Looking at commit ff20ccae9fdb, it seems that the documentation was
> intentionally written in terms of gigabytes rather than the number:
>
>> The threshold is two billion members, which was interpreted as 2GB
>> in the documentation. Fix to reflect that each member takes up five
>> bytes, which translates to about 10GB. This is not exact, because of
>> page boundaries. While at it, mention the maximum size 20GB.
>
> Anyway, I also think, as Ashutosh suggested, that if we want to fix
> this documentation, it would be better to do so in a separate patch.

Ah, I've found why you choose to add this doc modification in this patch
in the thread, sorry for skipping over the part:
| For earlier versions (18 and before), the storage-size approximation
| remains relevant because users don’t have direct access to member
| count information. Since we don’t plan to backpatch (I assume so) this
| new function, the documentation for older branches should continue to
| rely on the storage-based approximation.

| Now that pg_get_multixact_stats() exposes num_members, the HEAD branch
| docs can describe the thresholds in terms of counts directly.

Personally, I think it might be fine to keep the gigabyte-based
description, and perhaps we could show both the number of members and
gigabytes, since it'd be also helpful to have a sense of the scale.

--
Regards,

--
Atsushi Torikoshi
Seconded from NTT DATA Japan Corporation to SRA OSS K.K.