Re: Redesign per-backend statistics

Hi,

On Tue, Sep 22, 2026 at 09:16:30AM +0900, Michael Paquier wrote:
> On Mon, Sep 21, 2026 at 05:17:04AM -0400, Andres Freund wrote:
> > On 2026-09-21 07:15:44 +0000, Bertrand Drouvot wrote:
> >> +/* -------------------------------------------------------------------------
> >> + *
> >> + * pgstat_per_backend.c
> >> + * Generic infrastructure for per-backend statistics.
> >> + *
> >> + * This file manages the dedicated per-kind dshashes used for per-backend
> >> + * statistics, including entry creation, fetching, snapshots, transfer to
> >> + * global statistics, and removal.
> >
> > It's not entirely obvious to me that a hash table is needed here. If I
> > understand correctly, this just going to be used for per-backend versions of
> > fixed stats. Which means we could just as well allocate all the memory in
> > statically allocated shared memory at server start and have the per-backend
> > stats be accessible by nothing more than an array access?
>
> Yeah. Array access based on an procnum index is still feeling like
> the natural thing to do here (I think I've mentioned that upthread?).

Yeah you did mention it. I took your reply in [1] as agreeing with the concern
about reserving memory for mostly unused slots, though not necessarily with using
a dshash. Is your preference still for the array despite that memory cost?

[1]: https://postgr.es/m/anlVx_TD_Q8He5uf%40paquier.xyz

Regards,

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