Horton

Re: foreign key locks, 2nd attempt

Bruce Momjian <bruce@momjian.us>

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From: Bruce Momjian <bruce@momjian.us>
To: Alvaro Herrera <alvherre@commandprompt.com>
Cc: Simon Riggs <simon@2ndquadrant.com>, Robert Haas <robertmhaas@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Noah Misch <noah@leadboat.com>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2012-03-16T03:08:29Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Try to avoid running with a full fsync request queue.

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@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

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