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Re: foreign key locks, 2nd attempt

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>,Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>,Noah Misch <noah(at)leadboat(dot)com>,Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: foreign key locks, 2nd attempt
Date: 2012-03-16 03:04:06
Message-ID: 20120316030406.GA8738@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackers
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.

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

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

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