Re: Add pg_stat_kind_info system view

On Thu, Apr 30, 2026 at 07:38:57PM +0000, Tristan Partin wrote:
> Of course I forgot to attach the patch :D.

Thanks for the patch. That sounds like a good idea particularly for
track_entry_count, because we have no real way to provide this
information, which would be more valuable if an in-core stats kind has
the idea to switch this flag to true in the future, and I don't really
want all extensions to re-implement the same thing to access this
information.

+ if (info->track_entry_count)
+ {
+ values[2] = Int64GetDatum(pgstat_get_entry_count(kind));
+ }
+ else
+ {
+ nulls[2] = true;
+ }

Returning NULL if the flag is not set sounds sensible. For
fixed-numbered, fine by me for 1, so as it is possible to aggregate
the full size counting with the fixed shmem size of each stats kind.

+SELECT name, builtin FROM pg_stat_kind_info
+ ORDER BY name COLLATE "C";

This is not going to be stable if running installcheck on an instance
where a custom kind is loaded, so let's restrict the query to check
for built-in kinds.

I'd suggest to expand the data published to more fields and not only
what's presented here, so as it becomes possible to do more SQL sanity
checks with the stats kind info (same attribute name as the fields):
- fixed_amount, where shared_size > 0 does not make sense.
- snapshot_ctl_off and shared_ctl_off, that only makes sense under
fixed_amount. These should not be set for !fixed_amount.
- pending_size, that should not be set for fixed_amount.
- existence of flush_pending_cb, delete_pending_cb,
reset_timestamp_cb, to_serialized_name, from_serialized_name,
to_serialized_data, from_serialized_data (should be booleans), fine
for !fixed_amount, never for fixed_amount.
- existence of init_shmem_cb, reset_all_cb, snapshot_cb (should be
booleans), fine for fixed_amount, never for !fixed_amount.

I may be missing one or two things. pgstat_internal.h documents all
these requirements, the idea is to translate these requirements at SQL
level. We should definitely apply these checks for both custom and
built-in stats kinds, which should save time for developers of pgstats
in core and outside of core by detecting inconsistent patterns
beforehand.
--
Michael