Re: table_open/table_close with different lock mode

On Fri, Jul 21, 2023 at 2:57 PM Gurjeet Singh <gurjeet(at)singh(dot)im> wrote:
>
> On Thu, Jul 20, 2023 at 11:38 PM Junwang Zhao <zhjwpku(at)gmail(dot)com> wrote:
> >
> > On Fri, Jul 21, 2023 at 2:26 PM Michael Paquier <michael(at)paquier(dot)xyz> wrote:
> > >
> > > On Fri, Jul 21, 2023 at 02:05:56PM +0800, Junwang Zhao wrote:
> > > > I noticed there are some places calling table_open with
> > > > RowExclusiveLock but table_close with NoLock, like in function
> > > > toast_save_datum.
> > > >
> > > > Can anybody explain the underlying logic, thanks in advance.
> > >
> > > This rings a bell. This is a wanted behavior, see commit f99870d and
> > > its related thread:
> > > https://postgr.es/m/17268-d2fb426e0895abd4@postgresql.org
> > >
> >
> > I see this patch, so all the locks held by a transaction will be released
> > at the commit phase, right? Can you show me where the logic is located?
>
> The NoLock is simple a marker that tells the underlying machinery to
> not bother releasing any locks. As a matter of fact, you can pass
> NoLock in *_open() calls, too, to indicate that you don't want any new
> locks, perhaps because the transaction has already taken an
> appropriate lock on the object.
>
> As for lock-releasing codepath at transaction end, see
> CommitTransaction() in xact.c, and specifically at the
> ResourceOwnerRelease() calls in there.
>

Great, thanks for the thorough explanation, will look into the code :)

> Best regards,
> Gurjeet
> http://Gurje.et

--
Regards
Junwang Zhao