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

On Wed, Nov 19, 2025 at 08:10:43PM -0600, Sami Imseih wrote:
> It just occurred to me that the documentation [0] should be
> updated to describe the callbacks. I will do that in the next
> revision.
>
> [0] https://www.postgresql.org/docs/current/xfunc-c.html#XFUNC-ADDIN-CUSTOM-CUMULATIVE-STATISTICS

Hmm. Based on what I can read from the patch, you are still enforcing
file name patterns in the backend, as of:
+ extra->statfiles[i] = psprintf("%s/pgstat.%d.%d.stat",
+ PGSTAT_STAT_PERMANENT_DIRECTORY, kind, i);

My take (also mentioned upthread) is that this design should go the
other way around, where modules have the possibility to define their
own file names, and some of them could be generated on-the-fly when
writing the files, for example for a per-file database split, or the
object ID itself.

The important part for variable-numbered stats is that the keys of the
entries have to be in the main pgstats file. Then, the extra data is
loaded back based on the data in the entry key, based on a file name
that only a custom stats kind knows about (fd and file name). It
means that the custom stats kind needs to track the files it has to
clean up by itself in this scheme. We could pass back to the startup
process some fds that it cleans up, but it feels simpler here to let
the custom code do what they want, instead, rather than having an
array that tracks the file names and/or their fds.
--
Michael