Subject: Re: Get memory contexts of an arbitrary backend process
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, torikoshia@oss.nttdata.com
Cc: pgsql-hackers@postgresql.org, tgl@sss.pgh.pa.us,
 gkokolatos@protonmail.com, kasahara.tatsuhito@gmail.com,
 craig@2ndquadrant.com
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 <a8af0d22d87a9fd3f5983d3c6db55b03@oss.nttdata.com>
 <20210326.120104.2148406039277091336.horikyota.ntt@gmail.com>
From: Fujii Masao <masao.fujii@oss.nttdata.com>
Message-ID: <6738f309-a41b-cbe6-bb57-a1c58ce9f05a@oss.nttdata.com>
Date: Fri, 26 Mar 2021 12:26:31 +0900
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On 2021/03/26 12:01, Kyotaro Horiguchi wrote:
> At Thu, 25 Mar 2021 23:45:17 +0900, torikoshia <torikoshia@oss.nttdata.com> wrote in
>> Attached new one.

Regarding the argument max_children, isn't it better to set its default value,
e.g., 100 like MemoryContextStats() uses?

+		ereport(WARNING,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("must be a superuser to log memory contexts")));

IMO this type of error, i.e., permission error, should be treated as ERROR
rather than WARNING, like pg_terminate_backend() does.

+		ereport(WARNING,
+				(errmsg("failed to send signal: %m")));

Per message style rule, "could not send signal to process %d: %m" is better?

> Thanks!
> 
> -    SELECT * FROM pg_get_backend_memory_contexts();
> +    SELECT * FROM pg_get_backend_memory_contexts(0, 0);
> 
> However we can freely change the signature since it's new in 14, but I
> see the (void) signature as useful.  I vaguely thought of defining
> pg_get_backend_memory_contexts(void) in pg_proc.dat then defining
> (int, int) as a different function in system_views.sql.  That allows
> the function of the second signature to output nothing. You can make a
> distinction between the two signatures by using PG_NARGS() in the C
> function.
> 
> That being said, it's somewhat uneasy that two functions with the same
> name returns different values.  I'd like to hear others what they feel
> like about the behavior.

I think that it's confusing to merge two different features into one function.
Isn't it better to leave pg_get_backend_memory_contexts() as it is, and
make the log-memory-contexts feature as separate function, e.g.,
pg_log_backend_memory_contexts()?

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION