On Sun, May 16, 2010 at 9:07 PM, Florian Pflug <fgp@phlo.org> wrote:
> On May 14, 2010, at 22:54 , Robert Haas wrote:
>> On Thu, May 13, 2010 at 5:39 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> Florian Pflug <fgp@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
ofconflicts between FOR UPDATE locks and updates. With the patch, any attempt to UPDATE or FOR UPDATE lock a row that
hasconcurrently 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
assumptionthat a lock becomes irrelevant after the transaction(s) that held it completely is built deeply into the
multixact machinery that powers SHARE locks. That machinery therefore assumes that once all members of a multi xact
havecompleted the multi xact is dead also. But my proposal depends on a SERIALIZABLE transaction being able to find if
anyof the lockers of a row are invisible under it's snapshot - for which it'd need any multi xact containing invisible
xidsto 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.
https://commitfest.postgresql.org/action/commitfest_view/open
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company