Date: Wed, 11 Mar 2020 13:53:54 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Fujii Masao <masao.fujii@oss.nttdata.com>
Cc: Julien Rouhaud <rjuju123@gmail.com>,
	Peter Eisentraut <peter.eisentraut@2ndquadrant.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: backend type in log_line_prefix?
Message-ID: <20200311185354.GW29065@telsasoft.com>
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 <d7dd4aad-c0f3-ecc3-e0f9-def491951e2f@oss.nttdata.com>
 <20200310190142.GB29065@telsasoft.com>
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On Tue, Mar 10, 2020 at 02:01:42PM -0500, Justin Pryzby wrote:
> On Thu, Feb 13, 2020 at 06:43:32PM +0900, Fujii Masao wrote:
> > If we do this, backend type should be also included in csvlog?
> 
> +1, I've been missing that
> 
> Note, this patch seems to correspond to:
> b025f32e0b Add leader_pid to pg_stat_activity
> 
> I had mentioned privately to Julien missing this info in CSV log.
> 
> Should leader_pid be exposed instead (or in addition)?  Or backend_type be a

I looked more closely and played with the patch.

Can I suggest:

$ git diff
diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index 3a6f7f9456..56e0a1437e 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -2945,7 +2945,7 @@ write_csvlog(ErrorData *edata)
        if (MyProcPid == PostmasterPid)
                appendCSVLiteral(&buf, "postmaster");
        else if (MyBackendType == B_BG_WORKER)
-               appendCSVLiteral(&buf, MyBgworkerEntry->bgw_type);
+               appendCSVLiteral(&buf, MyBgworkerEntry->bgw_name);
        else
                appendCSVLiteral(&buf, pgstat_get_backend_desc(MyBackendType));


Then it logs the leader:
|2020-03-11 13:16:05.596 CDT,,,16289,,5e692ae3.3fa1,1,,2020-03-11 13:16:03 CDT,4/3,0,LOG,00000,"temporary file: path ""base/pgsql_tmp/pgsql_tmp16289.0"", size 4276224",,,,,,"explain analyze SELECT * FROM t a JOIN t b USING(i) WHERE i>999 GROUP BY 1;",,,"psql","parallel worker for PID 16210"

It'll be easy enough to extract the leader and join that ON leader=pid.

> I think this patch alone wouldn't provide that, and there'd need to either be a
> line logged for each worker.  Maybe it'd log full query+details (ugh), or just
> log "parallel worker of pid...".  Or maybe there'd be a new column with which
> the leader would log nworkers (workers planned vs workers launched - I would
> *not* want to get this out of autoexplain).

I'm still not sure how to do that, though.
I see I can get what's needed at DEBUG1:

|2020-03-11 13:50:58.304 CDT,,,16196,,5e692aa7.3f44,22,,2020-03-11 13:15:03 CDT,,0,DEBUG,00000,"registering background worker ""parallel worker for PID 16210""",,,,,,,,,"","postmaster"

But I don't think it's viable to run for very long with log_statement=all,
log_min_messages=DEBUG.

-- 
Justin