Re: How to expose session vs txn lock info in pg_locks view?

On Sun, 24 Jan 2021 at 09:12, Andres Freund <andres(at)anarazel(dot)de> wrote:

> Hi,
>
> On 2021-01-19 14:16:07 +0800, Craig Ringer wrote:
> > AFAICS it'd be necessary to expand PROCLOG to expose this in shmem.
> > Probably by adding a small bitfield where bit 0 is set if there's a txn
> > level lock and bit 1 is set if there's a session level lock. But I'm not
> > convinced that expanding PROCLOCK is justifiable for this.
> sizeof(PROCLOCK)
> > is 64 on a typical x64 machine. Adding anything to it increases it to 72
> > bytes.
>
> Indeed - I really don't want to increase the size, it's already a
> problem.
>
>
> > It's frustrating to be unable to tell the difference between
> session-level
> > and txn-level locks in diagnostic output.
>
> It'd be useful, I agree.
>
>
> > And the deadlock detector has no way to tell the difference when
> > selecting a victim for a deadlock abort - it'd probably make sense to
> > prefer to send a deadlock abort for txn-only lockers.
>
> I'm doubtful this is worth going for.
>
>
> > But I'm not sure I see a sensible way to add the info - PROCLOCK is
> > already free of any padding, and I wouldn't want to use hacks like
> > pointer-tagging.
>
> I think there's an easy way to squeeze out space: make groupLeader be an
> integer index into allProcs instead. That requires only 4 bytes...
>
> Alternatively, I think it'd be reasonably easy to add the scope as a bit
> in LOCKMASK - there's plenty space.
>

I was wondering about that, but concerned that there would be impacts I did
not understand.

I'm happy to pursue that angle.