Re: [HACKERS] Moving relation extension locks out of heavyweight lock manager

On Wed, 12 Feb 2020 at 00:43, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> I took a brief look through this patch. I agree with the fundamental
> idea that we shouldn't need to use the heavyweight lock manager for
> relation extension, since deadlock is not a concern and no backend
> should ever need to hold more than one such lock at once. But it feels
> to me like this particular solution is rather seriously overengineered.
> I would like to suggest that we do something similar to Robert Haas'
> excellent hack (daa7527af) for the !HAVE_SPINLOCK case in lmgr/spin.c,
> that is,
>
> * Create some predetermined number N of LWLocks for relation extension.

My original proposal used LWLocks and hash tables for relation
extension but there was a discussion that using LWLocks is not good
because it's not interruptible[1]. Because of this reason and that we
don't need to have two lock level (shared, exclusive) for relation
extension lock we ended up with implementing dedicated lock manager
for extension lock. I think we will have that problem if we use LWLocks.

Regards,

[1] https://www.postgresql.org/message-id/CA%2BTgmoZnWYQvmeqeGyY%2B0j-Tfmx8cTzRadfxJQwK9A-nCQ7GkA%40mail.gmail.com

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