Date: Wed, 23 Aug 2023 19:32:06 -0700
From: Nathan Bossart <nathandbossart@gmail.com>
To: Ian Lawrence Barwick <barwick@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: pg_stat_get_backend_subxact() and backend IDs?
Message-ID: <20230824023206.GA1128857@nathanxps13.lan>
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On Thu, Aug 24, 2023 at 10:22:49AM +0900, Ian Lawrence Barwick wrote:
> Looking at the code, this is happening because
> "pgstat_fetch_stat_local_beentry()"
> expects to be passed the backend ID as an integer representing a 1-based index
> referring to "localBackendStatusTable", but "pg_stat_get_backend_subxact()"
> is presumably intended to take the actual BackendId , as per other
> "pg_stat_get_XXX()"
> functions.

Yes, this was changed in d7e39d7, but 10ea0f9 seems to have missed the
memo.

> Assuming I am not misunderstanding something here (always a
> possibility, apologies
> in advance if this is merely noise), what is actually needed is a function which
> accepts a BackendId (as per "pgstat_fetch_stat_beentry()"), but returns a
> LocalPgBackendStatus (as per "pgstat_fetch_stat_local_beentry()") like the
> attached, clumsily named "pgstat_fetch_stat_backend_local_beentry()".

I think you are right.  The relevant information is only available in
LocalPgBackendStatus, but there's presently no helper function for
obtaining the "local" status with the BackendId.

> +LocalPgBackendStatus *
> +pgstat_fetch_stat_backend_local_beentry(BackendId beid)
> +{
> +	LocalPgBackendStatus key;
> +
> +	pgstat_read_current_status();
> +
> +	/*
> +	 * Since the localBackendStatusTable is in order by backend_id, we can use
> +	 * bsearch() to search it efficiently.
> +	 */
> +	key.backend_id = beid;
> +
> +	return (LocalPgBackendStatus *) bsearch(&key, localBackendStatusTable,
> +											localNumBackends,
> +											sizeof(LocalPgBackendStatus),
> +											cmp_lbestatus);
> +}

We could probably modify pgstat_fetch_stat_beentry() to use this new
function.  I suspect we'll want to work on the naming, too.  Maybe we could
name them pg_stat_fetch_local_beentry_by_index() and
pg_stat_fetch_local_beentry_by_backendid().

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com