Subject: Re: FOR SHARE vs FOR UPDATE locks
From: "Joshua D. Drake" <jd@commandprompt.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Heikki Linnakangas <heikki@enterprisedb.com>,
	Alvaro Herrera <alvherre@commandprompt.com>,
	Simon Riggs <simon@2ndquadrant.com>, pgsql-hackers@postgresql.org,
	pgsql-core@postgresql.org
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Date: Fri, 01 Dec 2006 10:54:38 -0800
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On Fri, 2006-12-01 at 13:46 -0500, Tom Lane wrote:
> "Heikki Linnakangas" <heikki@enterprisedb.com> writes:
> > Let's throw an error for now. We have to come back to this in 8.3, I think.
> 
> After further thought I think we should also seriously consider plan C:
> do nothing for now.  We now realize that there have been related bugs
> since 8.0, namely that
> 
> 	begin;
> 	select some rows for update;
> 	savepoint x;
> 	update the same rows;
> 	rollback to x;
> 
> leaves the tuple(s) not locked.  The lack of complaints about this from
> the field suggests that this isn't a huge problem in practice.  If we
> do make it throw an error I'm afraid that we will break applications
> that aren't having a problem at the moment.
> 
> I'm also realizing that a fix along the throw-an-error line is
> nontrivial, eg, HeapTupleSatisfiesUpdate would need another return code.
> 
> So at this point we are facing three options:
> 	- throw in a large and poorly tested "fix" at the last moment;
> 	- postpone 8.2 until we can think of a real fix, which might
> 	  be a major undertaking;
> 	- ship 8.2 with the same behavior 8.0 and 8.1 had.
> None of these are very attractive, but I'm starting to think the last
> is the least bad.

/me struggles...

IMHO:

option 2 is the correct option, but the least favorable.
option 1 is probably bad
option 3 is the lesser of the evils if we document it loudly.

Joshua D. Drake


> 
> 			regards, tom lane
> 
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