Add pg_stat_kind_info system view
Hey hackers,
In 7949d959458[0]https://github.com/postgres/postgres/commit/7949d9594582ab49dee221e1db1aa5401ace49d4, we introduced pluggable cumulative statistics.
Extensions could now register their own custom statistics. However,
there was no way for an extension author to introspect their custom
statistics through SQL, and I don't believe there is currently a way to
introspect builtin statistics either.
Additionally, there was no way to know how much shared memory each
statistics kind was using. We could only know the total shared memory
used by the statistics subsystem.
dbltap@postgres=# SELECT * FROM pg_shmem_allocations WHERE name = 'Shared Memory Stats';
name | off | size | allocated_size
---------------------+-----------+--------+----------------
Shared Memory Stats | 153456640 | 321976 | 321976
In this patch, I have added a new system view: pg_stat_kind_info built
on a new function pg_stat_get_kind_info(). The view has the following
columns:
- id: id of the statistics kind
- name: name of the statistics kind
- count: number of entries for the statistics kind
- builtin: whether the statistics kind of builtin or not
- shared_size: shared memory size of each entry
dbltap@postgres=# SELECT * FROM pg_stat_kind_info;
id | name | count | builtin | shared_size
----+-------------------------+--------+---------+-------------
1 | database | (null) | t | 288
2 | relation | (null) | t | 248
3 | function | (null) | t | 56
4 | replslot | (null) | t | 120
5 | subscription | (null) | t | 120
6 | backend | (null) | t | 2952
7 | archiver | 1 | t | 0
8 | bgwriter | 1 | t | 0
9 | checkpointer | 1 | t | 0
10 | io | 1 | t | 0
11 | lock | 1 | t | 0
12 | slru | 1 | t | 0
13 | wal | 1 | t | 0
25 | test_custom_var_stats | 0 | f | 40
26 | test_custom_fixed_stats | 1 | f | 56
(15 rows)
I am not sure that shared_size is a good column name, and I needed to
include pgstat_internal.h in pgstatfuncs.c to get everything working.
I'm also curious to hear if anyone thinks there is other valuable
information to expose.
[0]: https://github.com/postgres/postgres/commit/7949d9594582ab49dee221e1db1aa5401ace49d4
--
Tristan Partin
PostgreSQL Contributors Team
AWS (https://aws.amazon.com)
Of course I forgot to attach the patch :D.
--
Tristan Partin
PostgreSQL Contributors Team
AWS (https://aws.amazon.com)
Attachments:
v1-0001-Add-pg_state_kind_info-view-and-pg_stat_get_kind_.patchtext/x-patch; charset=utf-8; name=v1-0001-Add-pg_state_kind_info-view-and-pg_stat_get_kind_.patchDownload+229-2
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