Re: foreign key locks, 2nd attempt

On Thu, Mar 15, 2012 at 11:04:06PM -0400, Bruce Momjian wrote:
> On Tue, Mar 13, 2012 at 02:35:02PM -0300, Alvaro Herrera wrote:
> >
> > Excerpts from Bruce Momjian's message of mar mar 13 14:00:52 -0300 2012:
> > >
> > > On Tue, Mar 06, 2012 at 04:39:32PM -0300, Alvaro Herrera wrote:
> >
> > > > When there is a single locker in a tuple, we can just store the locking info
> > > > in the tuple itself. We do this by storing the locker's Xid in XMAX, and
> > > > setting hint bits specifying the locking strength. There is one exception
> > > > here: since hint bit space is limited, we do not provide a separate hint bit
> > > > for SELECT FOR SHARE, so we have to use the extended info in a MultiXact in
> > > > that case. (The other cases, SELECT FOR UPDATE and SELECT FOR KEY SHARE, are
> > > > presumably more commonly used due to being the standards-mandated locking
> > > > mechanism, or heavily used by the RI code, so we want to provide fast paths
> > > > for those.)
> > >
> > > Are those tuple bits actually "hint" bits? They seem quite a bit more
> > > powerful than a "hint".
> >
> > I'm not sure what's your point. We've had a "hint" bit for SELECT FOR
> > UPDATE for ages. Even 8.2 had HEAP_XMAX_EXCL_LOCK and
> > HEAP_XMAX_SHARED_LOCK. Maybe they are misnamed and aren't really
> > "hints", but it's not the job of this patch to fix that problem.
>
> Now I am confused. Where do you see the word "hint" used by
> HEAP_XMAX_EXCL_LOCK and HEAP_XMAX_SHARED_LOCK. These are tuple infomask
> bits, not hints, meaning they are not optional or there just for
> performance.

Are you saying that the bit is only a guide and is there only for
performance? If so, I understand why it is called "hint".

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +