Re: [Postgres-xc-developers] [postgres-xc:bugs] #454 Update triggers on replicated tables may corrupt data

[Pavan D. <pav...@gm...>]

On Wed, Oct 9, 2013 at 12:09 PM, Andrei Martsinchyk <
and...@gm...> wrote:

> I believe even plain vacuum may move tuples within a page and purge empty
> pages, therefore CTIDs could be changed.
>

Plain vacuum does not move tuples around.. Note that all indexes must be
updated if a tuple is moved, even within a page. But besides that all your
points below still hold true.


> Autovacuum on Datamodes are not in sync and concurrent writes may occur in
> different order on Datanodes.
> CTID can not be used to identify tuple replicas. Primary key could be
> used, or special system column. Otherwise not shippable updates against
> replicated tables should not be allowed.
>
>
> 2013/10/9 Pavan Deolasee <pav...@gm...>
>
>>
>>
>>
>> On Wed, Oct 9, 2013 at 11:49 AM, Koichi Suzuki <koi...@gm...>wrote:
>>
>>> Hmm.. Yes, it could be serious.   I suppose that CTID changes only when
>>> vacuum full occurs.
>>>
>>
>> CTID will also change every time a row is updated. So the bug should be
>> very easy to reproduce. One easiest way to compile different datanodes with
>> diferent block size. Another way could be to update/delete from a
>> replicated table and vacuum the table on one datanode but not other and
>> then again update several rows in the table. Most likely the new versions
>> on two different nodes will not have the same CTID anymore.
>>
>>
>>> Vacuum freeze will updates xmim to the smallest value and usual vacuum
>>> recycles "dead" row area.   We may have a chance to assign different CTIDs
>>> to the same rows in replicated table if vacuum full runs locally in
>>> datanodes.  (My assumption may be wrong.   Does vacuum freeze move tuples
>>> and change their CTID?)
>>>
>>>
>> No, it does not.
>>
>> Thanks,
>> Pavan
>>
>> --
>> Pavan Deolasee
>> http://www.linkedin.com/in/pavandeolasee
>>
>>
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>
>
> --
> Andrei Martsinchyk
>
>
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-- 
Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee