Thread: Re: pg_get_multixact_members not documented

On Fri, May 30, 2025 at 02:23:30PM -0500, Sami Imseih wrote:
> While looking into another multixact related topic, I realised that
> pg_get_multixact_members
> is not documented. I saw the lack of documentation was mentioned here [0], but
> this was never committed.
> 
> Any reason it should not be?

It looks to me like Álvaro just never got any feedback on the last patch he
sent.  I noticed that there are no in-tree uses, but I do see a couple of
blog posts that recommend it.  In any case, I can't think of a reason it
ought to remain undocumented.  Want to put together a patch?

-- 
nathan

On Fri, May 30, 2025 at 04:24:43PM -0500, Sami Imseih wrote:
>> Want to put together a patch?
> 
> Yes, will do

For extra credit, maybe we could add a test or two, too...

-- 
nathan

On Mon, Jun 02, 2025 at 12:46:51PM -0500, Sami Imseih wrote:
> v1-0001 is the documentation only patch. I improved upon the description
> suggested in [0]

Your patch adds an entry to the "Transaction ID and Snapshot Information
Functions" table, while Álvaro's introduced a new "Multixact Functions"
table.  His also added a note to maintenance.sgml.  Any reason for the
differences?

> A simple test will be a regress/sql which ensure the XID and lock mode
> of a transaction using a savepoint, something like the below. To do anything
> fancier with concurrency, we will need an isolation test.
> 
> ```
> drop table if exists t;
> create table t (v int);  insert into t values (1);
> begin;
> select from t for update ;
> savepoint s1;
> update t set v = v;
> select pg_get_multixact_members(a.relminmxid), a.relminmxid from
> (select relminmxid from pg_class where relname = 't') a;
> commit;
> ```

That seems reasonable to me.

-- 
nathan

v2 addresses the comments.

Adds a new section called "Multixact Information Functions" and a reference
to pg_get_multixact_members after the description of what multixact members
are in maintenance.sgml.

As I spent some time looking into this, I still think we should document this
function because of its use in blogs and examples that describe multixact.

However, this function does not appear to be very practical to use, because
the only visible MXID to the user is the oldest one, via pg_database.datminmxid
or pg_class.relfrozenxid, or with the help of the contrib extension pgrowlocks.

To actually find all multixact IDs and their associated XIDs that are yet to
be vacuumed, I could write a query like the following:

SELECT
b as mxid,
a.*
FROM
(SELECT min(datminmxid::text::int) min_datminmxid FROM pg_database) d,
generate_series(d.min_datminmxid, mxid_age(d.min_datminmxid::text::xid)) as b,
pg_get_multixact_members(b::text::xid) a

Another thing is in [0], we mention two things:

Running transactions and prepared transactions can be ignored if there is
no chance that they might appear in a multixact.

MXID information is not directly visible in system views such as
pg_stat_activity; however, looking for old XIDs is still a good way of
determining which transactions are causing MXID wraparound problems.

I really think this is an area that needs improvement. We tell users to ignore
transactions that have "no chance" of appearing in a multixact, but what that
really means is the user must somehow figure this out on their own. I don't
think it would be unrealistic to expose real-time information about backends
with transactions involved in multixacts.

Looking at a recent public discussion about multixact [1], it lists
"lack of direct visibility" as one of the areas for improvement. This
will be the
subject of a different thread, but I thought it would be good to mention it
here first as it's relevant.

[0] https://www.postgresql.org/docs/current/routine-vacuuming.html
[1] https://metronome.com/blog/root-cause-analysis-postgresql-multixact-member-exhaustion-incidents-may-2025

--
Sami Imseih
Amazon Web Services (AWS)