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.

Álvaro Herrera <alvhe...@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

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