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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: 2012-03-13 17:00:52
Message-ID: 20120313170052.GB9030@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, Mar 06, 2012 at 04:39:32PM -0300, Alvaro Herrera wrote:
> Here's a first attempt at a README illustrating this.  I intend this to
> be placed in src/backend/access/heap/README.tuplock; the first three
> paragraphs are stolen from the comment in heap_lock_tuple, so I'd remove
> those from there, directing people to this new file instead.  Is there
> something that you think should be covered more extensively (or at all)
> here?
...
> 
> 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".

-- 
  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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