Horton

Re: Pluggable cumulative statistics

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Michael Paquier <michael@paquier.xyz>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Postgres hackers <pgsql-hackers@lists.postgresql.org>
Date: 2024-07-04T20:56:52Z
Lists: pgsql-hackers 
Hi,

On 2024-07-03 18:47:15 +0900, Michael Paquier wrote:
> While looking at a different patch from Tristan in this area at [1], I
> still got annoyed that this patch set was not able to support the case
> of custom fixed-numbered stats, so as it is possible to plug in
> pgstats things similar to the archiver, the checkpointer, WAL, etc.
> These are plugged in shared memory, and are handled with copies in the
> stats snapshots.  After a good night of sleep, I have come up with a
> good solution for that, among the following lines:

> - PgStat_ShmemControl holds an array of void* indexed by
> PGSTAT_NUM_KINDS, pointing to shared memory areas allocated for each
> fixed-numbered stats.  Each entry is allocated a size corresponding to
> PgStat_KindInfo->shared_size.

I am dubious this is a good idea. The more indirection you add, the more
expensive it gets to count stuff, the more likely it is that we end up with
backend-local "caching" in front of the stats system.

IOW, I am against making builtin stats pay the price for pluggable
fixed-numbered stats.

It also substantially reduces type-safety, making it harder to refactor. Note
that you had to add static casts in a good number of additional places.

Greetings,

Andres Freund

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