Horton

Re: [Proposal] Adding callback support for custom statistics kinds

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: Peter Eisentraut <peter@eisentraut.org>
Cc: Sami Imseih <samimseih@gmail.com>, Chao Li <li.evan.chao@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2025-12-17T08:11:19Z
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. Fix const correctness in pgstat data serialization callbacks

  2. test_custom_stats: Add tests with read/write of auxiliary data

  3. Allow cumulative statistics to read/write auxiliary data from/to disk

  4. test_custom_stats: Test module for custom cumulative statistics

  5. injection_points: Remove portions related to custom pgstats

  6. Rename routines for write/read of pgstats file

Attachments

On Wed, Dec 17, 2025 at 08:03:36AM +0100, Peter Eisentraut wrote:
> So it seems to me that either the callbacks API needs some adjustments, or
> this particular implementation of the callback function is incorrect.

Hmm, you are right that this is not aligned.  This can be improved
with one change for each callback:
- It is OK with from_serialized_data() to manipulate the header data,
because we want to fill a portion of the shmem data with extra data
read from disk (the module wants to add a reference to a DSA stored in
the shmem entry, read from the second file).  So we should discard the
const marker from the callback definition.
- The const usage is OK for to_serialized_data(): it is better to
encourage a policy where the header data cannot be manipulated.  So
the const needs to be kept in the definition, but I also think that we
should change the module implementation so as the cast to
PgStatShared_CustomVarEntry is a const.

These changes result in the attached.  Sami, what do you think?
--
Michael