On Sun, May 16, 2010 at 9:07 PM, Florian Pflug <f...@phlo.org> wrote:
> On May 14, 2010, at 22:54 , Robert Haas wrote:
>> On Thu, May 13, 2010 at 5:39 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
>>> Florian Pflug <f...@phlo.org> writes:
>>>> All in all, I believe that SHARE and UPDATE row-level locks should be
>>>> changed to cause concurrent UPDATEs to fail with a serialization
>>>> error.
>>> I don't see an argument for doing that for FOR SHARE locks, and it
>>> already happens for FOR UPDATE (at least if the row actually gets
>>> updated).  AFAICS this proposal mainly breaks things, in pursuit of
>>> an unnecessary and probably-impossible-anyway goal of making FK locking
>>> work with only user-level snapshots.
>> After giving this considerable thought and testing the behavior at
>> some length, I think the OP has it right.  One thing I sometimes need
>> to do is denormalize a copy of a field, e.g.
>> <snipped example>
> I've whipped up a quick and still rather dirty patch that implements the 
> behavior I proposed, at least for the case of conflicts between FOR UPDATE 
> locks and updates. With the patch, any attempt to UPDATE or FOR UPDATE lock a 
> row that has concurrently been FOR UPDATE locked will cause a serialization 
> error. (The same for an actually updated row of course, but that happened 
> before too).
> While this part of the patch was fairly straight forward, make FOR SHARE 
> conflict too seems to be much harder. The assumption that a lock becomes 
> irrelevant after the transaction(s) that held it completely is built deeply 
> into the multi xact machinery that powers SHARE locks. That machinery 
> therefore assumes that once all members of a multi xact have completed the 
> multi xact is dead also. But my proposal depends on a SERIALIZABLE 
> transaction being able to find if any of the lockers of a row are invisible 
> under it's snapshot - for which it'd need any multi xact containing invisible 
> xids to outlive its snapshot.

Thanks for putting this together. I suggest adding it to the open
CommitFest - even if we decide to go forward with this, I don't
imagine anyone is going to be excited about changing it during beta.


Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company

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