Horton

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

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Cc: Naga Appani <nagnrik@gmail.com>, Kirill Reshke <reshkekirill@gmail.com>, pgsql-hackers@postgresql.org
Date: 2025-07-28T04:22:04Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Add pg_get_multixact_stats()

  2. Add MultiXactOffsetStorageSize() to multixact_internal.h

  3. Change GetMultiXactInfo() to return the next multixact offset

  4. Widen MultiXactOffset to 64 bits

  5. Refactor ReadMultiXactCounts() into GetMultiXactInfo()

  6. Move SQL-callable code related to multixacts into its own file

  7. Split func.sgml into more manageable pieces

On Fri, Jul 25, 2025 at 04:27:37PM +0530, Ashutosh Bapat wrote:
> In [1], we decided to document pg_get_multixact_member() in section
> "Transaction ID and Snapshot Information Functions". I think the
> discussion in the email thread applies to this function as well.

Yep, let's be consistent.

> Throwing an error causes the surrounding transaction to abort, so it
> should be avoided in a monitoring/reporting function if possible. In
> this case for example, we could throw a warning instead or report NULL
> values.

Most likely returning NULL is the best thing we can do, as a safe
fallback.

> The patch needs tests.

Indeed.

May I also suggest a split of the multixact SQL functions into a
separate file, a src/backend/utils/adt/multixactfuncs.c?  The existing
pg_get_multixact_members() relies on GetMultiXactIdMembers(),
available in multixact.h.  The new function pg_get_multixact_count()
relies on ReadMultiXactCounts(), which would mean adding it in
multixact.h.  Even if we finish without an agreement about the SQL
function and the end, publishing ReadMultiXactCounts() would give an
access to the internals to external code.

+PG_FUNCTION_INFO_V1(pg_get_multixact_count);

There should be no need for that, pg_proc.dat handling the
declaration AFAIK.

FWIW, these functions are always kind of hard to use for the end-user
without proper documentation.  You may want to add an example of how
one can use it for monitoring in the docs.
--
Michael