Re: Pluggable cumulative statistics

Hi,

On Thu, Jun 20, 2024 at 09:46:30AM +0900, Michael Paquier wrote:
> On Thu, Jun 13, 2024 at 04:59:50PM +0900, Michael Paquier wrote:
> > - How should the persistence of the custom stats be achieved?
> > Callbacks to give custom stats kinds a way to write/read their data,
> > push everything into a single file, or support both?
> > - Should this do like custom RMGRs and assign to each stats kinds ID
> > that are set in stone rather than dynamic ones?

> These two questions have been itching me in terms of how it would work
> for extension developers, after noticing that custom RMGRs are used
> more than I thought:
> https://wiki.postgresql.org/wiki/CustomWALResourceManagers
> 
> The result is proving to be nicer, shorter by 300 lines in total and
> much simpler when it comes to think about the way stats are flushed
> because it is possible to achieve the same result as the first patch
> set without manipulating any of the code paths doing the read and
> write of the pgstats file.

I think it makes sense to follow the same "behavior" as the custom
wal resource managers. That, indeed, looks much more simpler than v1.

> In terms of implementation, pgstat.c's KindInfo data is divided into
> two parts, for efficiency:
> - The exiting in-core stats with designated initializers, renamed as
> built-in stats kinds.
> - The custom stats kinds are saved in TopMemoryContext,

Agree that a backend lifetime memory area is fine for that purpose.

> and can only
> be registered with shared_preload_libraries.  The patch reserves a set
> of 128 harcoded slots for all the custom kinds making the lookups for
> the KindInfos quite cheap.

+                       MemoryContextAllocZero(TopMemoryContext,
+                                                          sizeof(PgStat_KindInfo *) * PGSTAT_KIND_CUSTOM_SIZE);

and that's only 8 * PGSTAT_KIND_CUSTOM_SIZE bytes in total.

I had a quick look at the patches (have in mind to do more):

> With that in mind, the patch set is more pleasant to the eye, and the
> attached v2 consists of:
> - 0001 and 0002 are some cleanups, same as previously to prepare for
> the backend-side APIs.

0001 and 0002 look pretty straightforward at a quick look.

> - 0003 adds the backend support to plug-in custom stats.

1 ===

It looks to me that there is a mix of "in core" and "built-in" to name the
non custom stats. Maybe it's worth to just use one?
 
As I can see (and as you said above) this is mainly inspired by the custom
resource manager and 2 === and 3 === are probably copy/paste consequences.

2 ===

+       if (pgstat_kind_custom_infos[idx] != NULL &&
+               pgstat_kind_custom_infos[idx]->name != NULL)
+               ereport(ERROR,
+                               (errmsg("failed to register custom cumulative statistics \"%s\" with ID %u", kind_info->name, kind),
+                                errdetail("Custom resource manager \"%s\" already registered with the same ID.",
+                                                  pgstat_kind_custom_infos[idx]->name)));

s/Custom resource manager/Custom cumulative statistics/

3 ===

+       ereport(LOG,
+                       (errmsg("registered custom resource manager \"%s\" with ID %u",
+                                       kind_info->name, kind)));

s/custom resource manager/custom cumulative statistics/

> - 0004 includes documentation.

Did not look yet.

> - 0005 is an example of how to use them, with a TAP test providing
> coverage.

Did not look yet.

As I said, I've in mind to do a more in depth review. I've noted the above while
doing a quick read of the patches so thought it makes sense to share them
now while at it.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com