Horton

Re: Pluggable cumulative statistics

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: Postgres hackers <pgsql-hackers@lists.postgresql.org>
Date: 2024-06-20T13:05:42Z
Lists: pgsql-hackers 
Hi,

On Thu, Jun 13, 2024 at 04:59:50PM +0900, Michael Paquier wrote:
> Hi all,
> 
> 2) Make the shmem pgstats pluggable so as it is possible for extensions
> to register their own stats kinds.

Thanks for the patch! I like the idea of having custom stats (it has also been
somehow mentioned in [1]).

> 2) is actually something that can be used for more things than
> just pg_stat_statements, because people love extensions and
> statistics (spoiler: I do).

+1

> * Making custom stats data persistent is an interesting problem, and
> there are a couple of approaches I've considered:
> ** Allow custom kinds to define callbacks to read and write data from
> a source they'd want, like their own file through a fd.  This has the
> disadvantage to remove the benefit of c) above.
> ** Store everything in the existing stats file, adding one type of
> entry like 'S' and 'N' with a "custom" type, where the *name* of the
> custom stats kind is stored instead of its ID computed from shared
> memory.

What about having 2 files?

- One is the existing stats file
- One "predefined" for all the custom stats (so what you've done minus the
in-core stats). This one would not be configurable and the extensions will
not need to know about it.

Would that remove the benefit from c) that you mentioned up-thread?

[1]: https://www.postgresql.org/message-id/20220818195124.c7ipzf6c5v7vxymc%40awork3.anarazel.de

Regards,

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

Commits

  1. injection_points: Add some fixed-numbered statistics

  2. injection_points: Add some cumulative stats for injection points

  3. Add helper routines to retrieve data for custom fixed-numbered pgstats

  4. Introduce pluggable APIs for Cumulative Statistics

  5. Switch PgStat_Kind from an enum to a uint32 type

  6. Improve comment of pgstat_read_statsfile()

  7. Add a new 'F' entry type for fixed-numbered stats in pgstats file

  8. Add PgStat_KindInfo.init_shmem_cb

  9. Use pgstat_kind_infos to write fixed shared statistics

  10. Replace hardcoded identifiers of pgstats file by #defines