Re: expose parallel leader in CSV and log_line_prefix

Hi,

On Fri, Jul 17, 2020 at 7:01 AM Michael Paquier <michael@paquier.xyz> wrote:
>
> On Thu, Jul 16, 2020 at 10:55:45PM -0400, Alvaro Herrera wrote:
> > Oh, ugh, I don't like that part much.  If you run connections through a
> > connection pooler, it's going to be everywhere. Let's put it there only
> > if the connection *is* running a parallel query, without being too
> > stressed about the startup and teardown sequence.
>
> Hmm.  Knowing if a leader is actually running parallel query or not
> requires a lookup at lockGroupMembers, that itself requires a LWLock.
> I think that it would be better to not require that.  So what if
> instead we logged %P only if Myproc has lockGroupLeader set and it
> does *not* match MyProcPid?  In short, it means that we would get the
> information of a leader for each worker currently running parallel
> query, but that we would not know from the leader if it is running a
> parallel query or not at the moment of the log.  One can then easily
> guess what was happening on the leader by looking at the logs of the
> backend matching with the PID the workers are logging with %P.

I had the same concern and was thinking about this approach too.
Another argument is that IIUC any log emitted due to
log_min_duration_statement wouldn't see the backend as executing a
parallel query, since the workers would already have been shut down.