Re: Row-level Locks & SERIALIZABLE transactions, postgres vs. Oracle

On May 17, 2010, at 3:30 , Robert Haas wrote:
> 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


Will do. Thanks for the link.

Here is an updated version that works for SHARE locks too.

(This message mainly serves as a way to link the updated patch to the commit fest entry. Is this how I'm supposed to do
that,or am I doing something wrong?) 

best regards,
Florian Pflug